Progress of a Comprehensive Familial Cancer Genetic Counseling Program in the Era of BRCA1 and BRCA2

BRCA1 and BRCA2 mutation carriers have an increased risk of developing breast and/or ovarian cancer. Technical advances in genetic testing have increased the need for genetic counseling services; therefore, we have developed a counseling program for these individuals. The purpose of this study is to characterize this population, assess level of interest in genetic testing, and evaluate our program over a 5-year period. Our Familial Cancer Genetic Counseling Program was established in November, 1994. Information was collected prospectively, with comprehensive evaluation including complete pedigree, risk assessment, and counseling by a genetic counselor, geneticist, and oncologist. Data were collected on risk level, and subsequent recommendations for screening and/or genetic testing. There were 824 contacts recorded from November, 1994, through August, 1999. To date, 162 families have undergone comprehensive genetic evaluation and counseling. 90 (56%) were seen for a concerning family history and 72 (44%) were seen due to a personal history of malignancy. The majority of families had a significant level of risk with 126 (78%) families having two and 70 (43%) families having three affected first-degree relatives. Of the 162 families who received full counseling, 125 (77%) met criteria to recommend BRCA1/BRCA2 genetic testing. At this time, 30 of the 162 (18%) have had genetic testing. A brief phone contact or clinic visit is useful to screen individuals so that counseling can be directed toward truly high-risk families. In our program, the majority of families counseled were eligible for BRCA1/BRCA2 testing, but only 18% have elected to proceed at this time.     

Impact of genetic testing for breast-ovarian cancer susceptibility

Previously, we have reported a clinical trial in which any woman in a defined geographic region who had a qualifying family history and who was referred by her physician or who was identified through a regional cancer registry was offered free genetic counseling, BRCA testing, and recommendations based on test results. Each family was represented by one affected and one unaffected person. Of the 87 families actually tested, 13 were found to have deleterious mutations. To assess the impact of the counseling and testing process, we contacted the tested individuals 1 month and 1 year after receiving the test result and those with an abnormal test result after 4 years. Index subjects, we found, differed significantly from relatives. Before coming for counseling, index subjects perceived both their general health and emotional health as worse than did their relatives. After counseling and testing, index subjects continue to worry more about breast cancer than do relatives. Affected subjects, we found, differed significantly from unaffected subjects. Before counseling, affected subjects knew more about breast cancer, perceived their general health as poorer, and reported greater adherence to recommended breast cancer surveillance than did unaffected subjects. After counseling and testing, affected subjects were less satisfied than unaffected subjects with having been tested. This study indicates that the group most prone to distress by cancer risk genetic counseling and testing is not the recruited relatives, nor even those affected with cancer, but rather the index patients themselves. The index patients, i.e., the ones who want the risk information most, appear to undergo the most stress in obtaining it.     

Factors associated with an individual's decision to withdraw from genetic testing for breast and ovarian cancer susceptibility: implications for counseling

Our study aimed to examine why individuals withdraw from genetic testing for breast and ovarian cancer susceptibility. We explored the characteristics of 334 individuals from high-risk breast and ovarian cancer families who declined genetic testing for BRCA1/2 mutations, when, and why they did so. Individuals who declined genetic testing were older, and a greater proportion had never developed breast or ovarian cancer. Fifty one per cent (51.1%) of individuals withdrew after the first genetic counseling session. Most of those who declined were afraid of the psychological effects of genetic testing (36.3%). The next most-cited explanations concerned logistic problems such as a limited ability to travel, lack of time, personal issues, advanced age, or health problems (21.7%). The third category included individuals who did not see any advantage in being tested (14.5%). Insurability was a concern (5.9%), mainly for men. Surprisingly, confidentiality was not a frequently reported issue (1.3%). Sixty eight per cent (68%) of individuals belonging to a family in which at least one individual has been tested withdrew after the presence of a deleterious BRCA1/2 mutation in a relative was disclosed, compared to 42% after the disclosure of a nonconclusive test result in at least one relative. Concern about the psychological effects of the result was still one of the major reasons. Several factors may influence an individual's decision to decline genetic testing; a greater understanding of these issues may help health professionals to better meet the needs and concerns of individuals from high-risk families, thus possibly improving their health outcomes.     

Management updates for women with a BRCA1 or BRCA2 mutation

In most cases of families with breast and ovarian cancer, the pattern of cancers in the family can be attributed to mutations in the BRCA1 and BRCA2 genes. Genetic testing for these cancer susceptibility genes typically takes place in the context of comprehensive genetic counseling. Strategies have been developed for the medical management of women at high risk of developing breast cancer, including options for screening and prophylactic surgery. BRCA1 and BRCA2 carriers are recommended to undergo prophylactic bilateral salpingo-oophorectomy by age 35-40 years or when childbearing is complete. This surgery significantly reduces the risk of ovarian cancer and also reduces the risk of breast cancer when performed in premenopausal mutation carriers. For breast cancer management, BRCA1 and BRCA2 carriers are offered the options of increased surveillance, with or without chemoprevention, or prophylactic surgery. Currently, BRCA carrier status is not used as an independent prognostic factor regarding systemic treatment options.     

Screening for 185delAG in the Ashkenazim.

A study was initiated to assess interest, educational effectiveness, and implications of genetic testing for the common BRCA1 mutation, 185delAG, in the Ashkenazim. Of 333 individuals who attended group sessions, 309 (92%) participated in the study. Participants were categorized as having negative family history (67%), positive family history (defined, by a relaxed criterion, as one first-degree relative or two second-degree relatives with breast [premenopausal] or ovarian cancer) (22%), positive personal history (7%), and both positive personal history and positive family history (4%). Group education was effective, as shown by the improvement in participant scores from pre- to posteducation tests. For the 289 individuals (94%) who requested testing, the major reasons included concern for their own risk, concern for the risk of their children, and desire to learn about surveillance options. The most common reason given by participants who declined testing was concern about health insurance. Six participants found to be heterozygous for the 185delAG mutation received results and were offered genetic counseling. Participants had consented for additional testing without receiving results and were screened for the 6174delT mutation in BRCA2, and seven were found to be positive. All identified carriers reported at least one first- or second-degree relative with a history of breast or ovarian cancer, although they did not all meet our study criteria for positive family history. Given these outcomes, we conclude that screening for breast and ovarian cancer susceptibility is most appropriate for individuals with a positive personal or positive family cancer history. We propose a guideline for future studies designed to identify individuals who may benefit from genetic testing for inherited breast and ovarian cancer.     

A computer model to simulate family history of breast/ovarian cancer in BRCA1 mutation carriers

The BRCA1 gene and its relationship to family history of breast/ovarian cancer are difficult to study in a population because of practical and ethical issues. The paucity of information on BRCA1 in the general population was a major theme in a recent review of genetic testing in Canada. We develop a simulation model to mimic genetic inheritance and cancer incidence in the family of someone with a germline BRCA1 mutation. Given someone's age and family structure, our model simulates his or her family history in three steps: (1) determine which family members have the mutation, (2) determine the ages of family members and (3) determine which family members have breast/ovarian cancer. Each step involves random variation. Some parameters in our model are estimated using local (British Columbia, Canada) population data. The breast/ovarian cancer risk associated with BRCA1 mutations is estimated using values published in the literature. An example is provided to illustrate the model's application. The model incorporates results from genetics, demography and epidemiology, but requires several additional assumptions. Research to address these assumptions is recommended.     

Detection of eight BRCA1 mutations in 10 breast/ovarian cancer families, including 1 family with male breast cancer.

Genetic epidemiological evidence suggests that mutations in BRCA1 may be responsible for approximately one half of early onset familial breast cancer and the majority of familial breast/ovarian cancer. The recent cloning of BRCA1 allows for the direct detection of mutations, but the feasibility of presymptomatic screening for cancer susceptibility is unknown. We analyzed genomic DNA from one affected individual from each of 24 families with at least three cases of ovarian or breast cancer, using SSCP assays. Variant SSCP bands were subcloned and sequenced. Allele-specific oligonucleotide hybridization was used to verify sequence changes and to screen DNA from control individuals. Six frameshift and two missense mutations were detected in 10 different families. A frameshift mutation was detected in a male proband affected with both breast and prostate cancer. A 40-bp deletion was detected in a patient who developed intra-abdominal carcinomatosis 1 year after prophylactic oophorectomy. Mutations were detected throughout the gene, and only one was detected in more than a single family. These results provide further evidence that inherited breast and ovarian cancer can occur as a consequence of a wide array of BRCA1 mutations. These results suggests that development of a screening test for BRCA1 mutations will be technically challenging. The finding of a mutation in a family with male breast cancer, not previously thought to be related to BRCA1, also illustrates the potential difficulties of genetic counseling for individuals known to carry mutations.     

Determining carrier probabilities for breast cancer-susceptibility genes BRCA1 and BRCA2.

Breast cancer-susceptibility genes BRCA1 and BRCA2 have recently been identified on the human genome. Women who carry a mutation of one of these genes have a greatly increased chance of developing breast and ovarian cancer, and they usually develop the disease at a much younger age, compared with normal individuals. Women can be tested to see whether they are carriers. A woman who undergoes genetic counseling before testing can be told the probabilities that she is a carrier, given her family history. In this paper we develop a model for evaluating the probabilities that a woman is a carrier of a mutation of BRCA1 and BRCA2, on the basis of her family history of breast and ovarian cancer in first- and second-degree relatives. Of special importance are the relationships of the family members with cancer, the ages at onset of the diseases, and the ages of family members who do not have the diseases. This information can be elicited during genetic counseling and prior to genetic testing. The carrier probabilities are obtained from Bayes's rule, by use of family history as the evidence and by use of the mutation prevalences as the prior distribution. In addressing an individual's carrier probabilities, we incorporate uncertainty about some of the key inputs of the model, such as the age-specific incidence of diseases and the overall prevalence of mutations. There is some evidence that other, undiscovered genes may be important in explaining familial breast cancer. Users of the current version of the model should be aware of this limitation. The methodology that we describe can be extended to more than two genes, should data become available about other genes.     

Motivations and concerns of women considering genetic testing for breast cancer: a comparison between affected and at-risk probands

Since the discovery of the BRCA1 and BRCA2 genes, there has been an increasing demand for breast cancer risk assessment programs. In an effort to understand and serve the population such programs target better, several studies have identified factors influencing high-risk women to pursue breast cancer risk assessment and genetic testing services; none, however, has focused on how the motivations and concerns of at-risk women may differ from their previously affected counterparts, who are typically the initial members of their families to undergo genetic testing. The majority of both previously affected and unaffected women felt that preventative surgery decisions, surveillance practices, the assessment of children's risks, and increased breast cancer anxiety were "more important" or "very important" issues regarding their thoughts about genetic testing. Significantly more affected women deemed family members' opinions "more" or "very important" (p < 0.01). Opinions concerning insurance and employment discrimination did not vary significantly between groups; however, a larger percentage of affected women felt this issue was of importance. Although all issues above should be addressed with women seeking cancer risk assessment and genetic testing, this research may help health care providers to gain a greater understanding of how the motivators and concerns of high-risk women can differ with personal cancer status so that referral, counseling, and education can be executed optimally.     

First BRCA1 and BRCA2 gene testing implemented in the health care system of Stockholm

The aim of the study was to optimize the criteria for the BRCA1 and BRCA2 gene testing and to improve oncogenetic counseling in the Stockholm region. Screening for inherited breast cancer genes is laborious and a majority of tested samples turn out to be negative. The frequencies of mutations in the BRCA1 and BRCA2 genes differ across populations. Between 1997 and 2000, 160 families with breast and/or ovarian cancer were counseled and screened for mutations in the two genes. Twenty-five BRCA1 and two BRCA2 disease-causing mutations were found. Various factors associated with the probability of finding a BRCA1 mutation in the families were estimated. Age of onset in different generations and other malignancies were also studied. Families from our region in which both breast and ovarian cancer occur were likely to carry a BRCA1 mutation (34%). In breast-only cancer families, mutations were found only in those with very early onset. All breast- only cancer families with a mutation had at least one case of onset before 36 years of age and a young median age of onset (<43 years). Other malignancies than breast and ovarian cancers did not segregate in the BRCA1 families and surveillance for other malignancies is not needed, in general. Decreasing age of onset with successive generations was common and must be taken into account when surveillance options are considered.     

Factors determining dissemination of results and uptake of genetic testing in families with known BRCA1/2 mutations

BACKGROUND: Uptake of genetic testing remains low, even in families with known BRCA1 and BRCA2 (BRCA1/2) mutations, despite effective interventions to reduce risk. We report disclosure and uptake patterns by BRCA1/2-positive individuals to at-risk relatives, in the setting of no-cost genetic counseling and testing. METHODS: Relatives of BRCA1/2-positive individuals were offered cost-free and confidential genetic counseling and testing. If positive for a BRCA1/2 mutation, participants were eligible to complete a survey about their disclosure of mutation status and the subsequent uptake of genetic testing by at-risk family members. RESULTS: One hundred and fifteen of 142 eligible individuals responded to the survey (81%). Eighty-eight (77%) of those surveyed disclosed results to all at-risk relatives. Disclosure to first-degree relatives (FDRs) was higher than to second-degree relatives (SDRs) and third-degree relatives (TDR) (95% vs. 78%; p < 0.01). Disclosure rates to male versus female relatives were similar, but reported completion of genetic testing was higher among female versus male FDRs (73% vs. 49%; p < 0.01) and SDRs (68% vs. 43%; p < 0.01), and among members of maternal versus paternal lineages (63% vs. 0%; p < 0.01). Men were more likely than women to express general difficulty discussing positive BCRA1/2 results with at-risk family members (90% vs. 70%; p = 0.03), while women reported more emotional distress associated with disclosure than men (48% vs. 13%; p < 0.01). DISCUSSION: We report a very high rate of disclosure of genetic testing information to at-risk relatives. However, uptake of genetic testing among at-risk individuals was low despite cost-free testing services, particularly in men, SDRs, and members of paternal lineages. The complete lack of testing among paternally related at-risk individuals and the lower testing uptake among men signify a significant barrier to testing and a challenge for genetic counselors and physicians working with high-risk groups. Further research is necessary to ensure that family members understand their risk and the potential benefits of genetic counseling.     

The role of financial factors in acceptance of clinical BRCA genetic testing

Many women who are offered BRCA genetic testing by genetics professionals do not have the test, possibly for financial reasons. We explored financial factors implicated in non-uptake of BRCA testing in women who had received genetic counseling in a clinical setting. Specifically, we described financial factors (affordability, health insurance, other) involved with BRCA testing; compared nonfinancial factors (disease, sociodemographic, risk assessment) in women who did not have BRCA testing (nontesters) with women who had the test (testers); showed associations of financial and nonfinancial factors with BRCA testing; and identified predictors of non-uptake of BRCA testing. The sample of 100 women (64 nontesters and 36 testers) completed an anonymous mailed survey on financial factors; 52 of the nontesters answered questions about nonfinancial factors. Testers had significantly better affordability and insurance coverage (p < 0.001), more diagnoses of breast or ovarian cancer (p < 0.05) and higher rates of receiving post-counseling risk estimates (p < 0.05), than nontesters. Non-uptake was 5.5-fold more likely in women that could not afford full or partial payment for the test and was 15.5-fold more likely in women that did not recall receiving risk estimates post-counseling. For many women having risk factors for breast/ovarian cancer, affordability of BRCA testing and insurance coverage for the test remain problematic. Post-counseling reminders of risk estimates may contribute to uptake of testing.     

Non-uptake of predictive genetic testing for BRCA1/2 among relatives of known carriers: attributes, cancer worry, and barriers to testing in a multicenter clinical cohort

BRCA1/2 test decliners/deferrers have received almost no attention in the literature and this is the first study of this population in the United Kingdom. The aim of this multicenter study is to examine the attributes of a group of individuals offered predictive genetic testing for breast/ovarian cancer predisposition who did not wish to proceed with testing at the time of entry into this study. This forms part of a larger study involving 9 U.K. centers investigating the psychosocial impact of predictive genetic testing for BRCA1/2. Cancer worry and reasons for declining or deferring BRCA1/2 predictive genetic testing were evaluated by questionnaire following genetic counseling. A total of 34 individuals declined the offer of predictive genetic testing. Compared to the national cohort of test acceptors, test decliners are significantly younger. Female test decliners have lower levels of cancer worry than female test acceptors. Barriers to testing include apprehension about the result, traveling to the genetics clinic, and taking time away from work/family. Women are more likely than men to worry about receiving less screening if found not to be a carrier. The findings do not indicate that healthy BRCA1/2 test decliners are a more vulnerable group in terms of cancer worry. However, barriers to testing need to be discussed in genetic counseling.     

Screening for genomic rearrangements in families with breast and ovarian cancer identifies BRCA1 mutations previously missed by conformation-sensitive gel electrophoresis or sequencing

The frequency of genomic rearrangements in BRCA1 was assessed in 42 American families with breast and ovarian cancer who were seeking genetic testing and who were subsequently found to be negative for BRCA1 and BRCA2 coding-region mutations. An affected individual from each family was tested by PCR for the exon 13 duplication (Puget et al. 1999a) and by Southern blot analysis for novel genomic rearrangements. The exon 13 duplication was detected in one family, and four families had other genomic rearrangements. A total of 5 (11. 9%) of the 42 families with breast/ovarian cancer who did not have BRCA1 and BRCA2 coding-region mutations had mutations in BRCA1 that were missed by conformation-sensitive gel electrophoresis or sequencing. Four of five families with BRCA1 genomic rearrangements included at least one individual with both breast and ovarian cancer; therefore, 4 (30.8%) of 13 families with a case of multiple primary breast and ovarian cancer had a genomic rearrangement in BRCA1. Families with genomic rearrangements had prior probabilities of having a BRCA1 mutation, ranging from 33% to 97% (mean 70%) (Couch et al. 1997). In contrast, in families without rearrangements, prior probabilities of having a BRCA1 mutation ranged from 7% to 92% (mean 37%). Thus, the prior probability of detecting a BRCA1 mutation may be a useful predictor when considering the use of Southern blot analysis for families with breast/ovarian cancer who do not have detectable coding-region mutations.     

On doing ‘being ordinary’: women's accounts of BRCA testing and maternal responsibility

Abstract

In recent months, genetic testing for the breast and ovarian cancer genes, BRCA1 and BRCA2, has again hit the headlines in the UK press. Four women, all from families with a strong breast cancer pedigree, have asked the Human Fertility and Embryology Authority for permission to screen their embryos for BRCA1/2 mutations. Although this may be a dramatic example of parental responsibility, other studies have shown that women at risk of BRCA-related cancers frequently cite responsibility to others as an important influence on their testing and treatment decision making. In this paper, I explore the decision-making explanations that women at risk of BRCA-related breast and ovarian cancer provide when accounting for their decision to undergo genetic testing. In doing so, I treat women's accounts critically, and examine how and why the women verbalize their explanations in the manner that they do.     

First recurrent large genomic rearrangement in the BRCA1 gene found in Poland

Mutation in the BRCA1 gene increases the risk of the person developing breast and/or ovarian cancer. The prevalence and spectrum of large genomic rearrangements (LGRs) varies considerably among different tested populations. In our previous study we described three LGRs in BRCA1 (exons 13–19, exon 17 and exon 22) in Polish families at high risk of breast and ovarian cancer. In this study we analyzed a group of 550 unselected women with ovarian cancer for the three previously identified LGRs. We used a rapid, single-step and closed-tube method: high-resolution melting analysis (HRMA). In this group of unrelated patients diagnosed with ovarian cancer we found three cases with the same deletions of exon 22. This is the first recurrent large deletion in BRCA1 found in Poland. We conclude that screening for the exon 22 deletion in BRCA1 should be included in the Polish BRCA1 genetic testing panel and possibly extended into other Slavic populations.     

BRCA1 maps proximal to D17S579 on chromosome 17q21 by genetic analysis

Previous studies have demonstrated linkage between early-onset breast cancer and ovarian cancer and genetic markers on chromosome 17q21. These markers define the location of a gene (BRCA1) which appears to be inherited as an autosomal dominant susceptibility allele. We analyzed five families with multiple affected individuals for evidence of linkage to the BRCA1 region. Two of the five families appear to be linked to BRCA1. One apparently linked family contains critical recombinants, suggesting that the gene is proximal to the marker D17S579 (Mfd188). These findings are consistent with the maximum-likelihood position estimated by the Breast Cancer Linkage Consortium and with recombination events detected in other linked families. Linkage analysis was greatly aided by PCR-based analysis of paraffin-embedded normal breast tissue from deceased family members, demonstrating the feasibility and importance of this approach. One of the two families with evidence of linkage between breast cancer and genetic markers flanking BRCA1 represents the first such family of African-American descent to be reported in detail.     

Breast and ovarian cancer incidence in BRCA1-mutation carriers. Breast Cancer Linkage Consortium.

Dominant predisposition to early-onset breast cancer and/or ovarian cancer in many families is known to be the result of germ-line mutations in a gene on chromosome 17q, known as BRCA1. In this paper we use data from families with evidence of linkage to BRCA1 to estimate the age-specific risks of breast and ovarian cancer in BRCA1-mutation carriers and to examine the variation in risk between and within families. Under the assumption of no heterogeneity of risk between families, BRCA1 is estimated to confer a breast cancer risk of 54% by age 60 years (95% confidence interval [CI] 27%-71%) and an ovarian cancer risk of 30% by age 60 years (95% CI 8%-47%). Similar lifetime-risk estimates are obtained by examining the risks of contralateral breast cancer and of ovarian cancer, in breast cancer cases in linked families. However, there is significant evidence of heterogeneity of risk between families; a much better fit to the data is obtained by assuming two BRCA1 alleles, one conferring a breast cancer risk of 62% and an ovarian cancer risk of 11% by age 60 years, the other conferring a breast cancer risk of 39% and an ovarian cancer risk of 42%, with the first allele representing 71% of all mutations (95% CI 55%-87%). There is no evidence of clustering of breast and ovarian cancer cases within families.     

Evaluation of the needs of male carriers of mutations in BRCA1 or BRCA2 who have undergone genetic counseling

To date, the concerns of men at risk of inheriting a BRCA1 mutation or a BRCA2 mutation have received little attention. It had been anticipated that few men would be interested in predictive testing when a BRCA mutation was identified in their family. However, these men are often affected emotionally by diagnoses of breast cancer in their relatives and may themselves harbor fears that cancer will develop. Male carriers of BRCA1/2 mutations are at increased risk of development of cancers of several types, including those of the breast and prostate. We conducted an evaluation of the needs and experiences of 59 male carriers of BRCA1/2 mutations followed at either the University of Toronto or Creighton University. We assessed their motivations for seeking genetic counseling and testing, involvement in family discussions of breast and ovarian cancer, risk perception, changes in cancer-screening practices, and overall satisfaction with the genetic-counseling process. The principal motivation for seeking genetic counseling was concern for their daughters. The majority (88%) of men participated in family conversations about breast and ovarian cancer, and 47% participated in conversations about prophylactic surgery. Most men believed that they were at increased risk of development of cancer (prostate, breast, colorectal, and skin cancers). However, fewer than one-half (43%) of the men with no previous diagnosis of cancer stated that their prostate cancer-surveillance practices had changed after they had received genetic test results. More than one-half (55%) had intrusive thoughts about their cancer risk. Although levels of satisfaction were high, practitioners should be aware of (a) potential pressures influencing men to request predictive testing, (b) the difficulties that men encounter in establishing surveillance regimens for breast and prostate cancer, and (c) the general lack of information about men's particular experiences in the medical community.     

No Evidence of BRCA1/2 genomic rearrangements in high-risk French-Canadian breast/ovarian cancer families

The discovery of deleterious mutations in the breast and ovarian cancer susceptibility genes, BRCA1 and BRCA2, has facilitated the identification of individuals at particularly high risk of these diseases. There is a wide variation between populations in the prevalence and related risks of various types of BRCA1/2 mutations, so estimates cannot be extrapolated to Canadians, especially not founder populations such as French- Canadians. Polymerase chain reaction (PCR)-based methods were used to detect the majority of these mutations. These approaches usually failed to detect large DNA rearrangements, which have been claimed to be involved in other populations in 5% to up to 36% of BRCA1-positive families. There is very little information about the contribution of this type of mutation in BRCA2-positive families. To investigate if our available mutation spectrum of BRCA1 and BRCA2 in high-risk French-Canadian breast/ovarian cancer families has been biased by PCR-based direct sequencing methods, we first used Southern blot analysis to test DNA samples from 61 affected/obligate carrier individuals from 58 families in which no BRCA1/2 deleterious mutation was found. Finally, 154 individuals from 135 BRCA1/2 nonconclusive families, including all those tested previously by Southern blot analysis, were tested with the new multiplex ligation probe amplification (MLPA) technique. These approaches failed to detect any rearrangement. Moreover, if the frequency of MLPA-detectable rearrangements in our cohort of 135 BRCA1/2 nonconclusive families was 2.2% or higher, we would have had a 95% or greater chance of observing at least one such rearrangement. As no rearrangements were identified, such large rearrangements must be quite rare in our population.