Complete and rapid scanning of the cystic fibrosis transmembrane conductance regulator (CFTR) gene by denaturing high-performance liquid chromatography (D-HPLC): major implications for genetic counselling

More than 900 mutations and more than 200 different polymorphisms have now been reported in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Ten years after the cloning of the CFTR gene, the complete scanning of the 27 exons to identify known and novel mutations remains challenging. Rapid accurate identification of mutated alleles is important for prenatal diagnosis, for cascade screening in families at risk of cystic fibrosis (CF) and for understanding the correlation between genotype and phenotype. In this study, we report the successful use of denaturing ion-pair reverse-phase high performance liquid chromatography (D-HPLC) to analyse rapidly the complete coding sequence of the CFTR gene. With 27 pairs of polymerase chain reaction primers, we optimised the temperature conditions required for the analysis of each amplicon and validated thetest conditions on samples from a panel of 1552 CF patients who came from France and other European countries and who had mutations and polymorphisms located in the various melting domains of the gene. D-HPLC identified 415 mutated alleles previously characterised by denaturing gradient gel electrophoresis and DNA sequencing, plus 74 novel mutations reported here.This new technique for screening DNA for sequence variation was extremely accurate (it identified 100% of the CFTR alleles tested so far) and rapid (the complete CFTR gene could be analysed in less than a week). Our approach should reduce the number of untyped CF alleles in populations and thus decrease the residual risk in couples at risk of CF. This technique may be important not only for CF,but also for many other genes with a high frequency of point mutations at a variety of sites.     

Retrospective study of the cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations in Guthrie cards from a large cohort of neonatal screening for cystic fibrosis

The cystic fibrosis transmembrane conductance regulator (CFTR) gene encodes a cAMP-activated chloride channel, and in individuals with both alleles of the gene mutated, symptoms of CF disease are manifest. With more than 300 mutations so far described in the gene the profile of mutant alleles in a population is specific to its ethnic origin. For an analysis with an unbiased recruitment of the CF alleles in neonates of similar origin (Normandy, France), we have retrospectively analyzed the Guthrie cards of affected newborns, diagnosed by the immunoreactive trypsinogen (IRT) assay. Analysis of the 27 exons of the CFTR gene using a GC clamp denaturing gradient gel electrophoresis (DGGE) assay has enabled us to identify over 96% of the mutated alleles. Two of these were novel mutations. We would like to propose this strategy as an efficient method of retrospective molecular genetic diagnosis that can be performed wherever Guthrie cards can be obtained. Knowledge of rare alleles could be a prerequisite for CF therapy in the future.     

Microsatellite haplotypes associations with 5 CFTR mutations in "Grande Brière", an isolate located in southern Brittany

The variability at three microsatellites in the Cystic Fibrosis Transmembrane Conductance Regulator Gene (CFTR) locus has been studied for frequent mutations encountered in an isolated population of "Grande Brière", a small region located in Southern Brittany. Fluorescent multiplex PCR of these microsatellites were assayed in 16 Cystic Fibrosis (CF) families carrying 5 different mutations. The four most frequent haplotypes on df508 chromosomes were the same as those found in Northern France and Europe but the distribution of these haplotypes provides new enlightenment on the population origin of this insular community.     

CFTR Haplotype Analysis Reveals Genetic Heterogeneity in the Etiology of Congenital Bilateral Aplasia of the Vas Deferens

Congenital bilateral aplasia of the vas deferens (CBAVD) was suggested to be a mild form of cystic fibrosis (CF). Mutation analysis of the cystic fibrosis transmembrane conductance regulator (CFTR) gene in males with CBAVD revealed that in some males CBAVD is caused by two defective CFTR alleles. The genetic basis of CBAVD in the other males and its association with CF remained unclear. We undertook this study to test the hypothesis of commonality of CBAVD and CF by haplotype analysis, in the CFTR locus, of males suffering from CBAVD and of their families. According to the hypothesis of commonality of CBAVD and CF, two brothers with CBAVD are expected to carry the same two CFTR alleles, while their fertile brothers are expected to carry at least one different allele. Eleven families were studied, of which two families, with unidentified CFTR mutations, did not support this hypothesis. In these families two brothers with CBAVD inherited different CFTR alleles. Their fertile brothers inherited the same CFTR alleles as their brothers with CBAVD. These results provide evidence for genetic heterogeneity in CBAVD. Though in some families CBAVD is associated with two CFTR mutations, we suggest that in others it is caused by other mechanisms, such as mutations at other loci or homozygosity or heterozygosity for partially penetrant CFTR mutations.     

Analysis of the CFTR gene in Turkish cystic fibrosis patients: identification of three novel mutations (3172delAC, P1013L and M1028I)

In order to determine the spectrum of cystic fibrosis (CF) mutations in the Turkish population, a complete coding region of the cystic fibrosis transmembrane conductance regulator (CFTR) gene including exon-intron boundaries, on 122 unrelated CF chromosomes from 73 Turkish CF families was analysed by denaturing gradient gel electrophoresis and multiplex heteroduplex analysis on MDE gel matrix. In addition to 15 previously reported mutations and 12 polymorphisms, three novel mutations, namely 3172delAC, P1013L and M1028I, were detected. ΔF508 was found to be present on 18.8% of CF chromosomes. The second most common mutation was 1677delTA, with a frequency of 7.3%, followed by G542X and 2183AA→G mutations, with frequencies of 4.9%. These four most common mutations in Turkish CF population account for approximately 36% of mutations. This study could only detect 52.5% of disease-causing mutations in this population; 47.5% of CF alleles remain to be identified, reflecting the high molecular heterogeneity of the Turkish population. Received: 16 June 1997 / Accepted: 18 September 1997     

Analysis of cystic fibrosis gene mutations and associated haplotypes in the Croatian population

The aim of this study was to reveal the CFTR gene mutation status in the Croatian population as well as to establish the haplotypes associated with cystic fibrosis (CF) and those associated with specific gene mutations. A total of 48 unrelated CF patients from Croatia were examined. Among 96 tested alleles, we found nine different mutations: DeltaF508, 58.33%; G542X, 3.12%; N1303K, 2.08%; R1162X; 621 + 1G --> T; G85E; Y569C; E585X; and S466X, 1.04%. Analysis of three polymorphic loci revealed 15 different haplotypes. Two of them (21-23-13 and 21-17-13) occurred with a higher frequency (40% and 24%). Both of these haplotypes also carried a CFTR gene mutation (DeltaF508 or G542X) on 27 out of 32 chromosomes. Among 12 (of all together 29) CF alleles on which no mutations were found, we detected 10 different haplotypes. Because there are still no published data on the distribution of polymorphic loci in Croatia, nor haplotypes associated with mutations in the CFTR gene, our results greatly contribute to knowledge regarding the genetic background of CF in this region.     

XV-2c and KM.19 haplotype analysis in Chilean patients with cystic fibrosis and unknown CFTR gene mutations

Cystic fibrosis (CF) is caused by mutations in the CFTR gene. More than 1600 mutations have been described, with frequencies that differ worldwide according to the ethnic origin of patients. A small group of mutations are recurrent on several populations. It has been shown that they each tend occur on specific chromosome 7 haplotypes, supporting the notion of a single origin for them. Less than 50% of mutations in Chilean patients have been identified to date. To indirectly assess the possible presence of a predominant founder mutation in the remaining unknown alleles, we evaluated 2 polymorphic markers, XV-2c and KM.19, tightly linked to the CFTR locus. The study was done in Chilean CF patients with unknown or deltaF508 (DeltaF508) CFTR mutations and their haplotypes were compared to affected family-based controls. DeltaF508 showed marked linkage disequilibrium with XV-2c/KM.19 haplotype B, with 90% of alleles on that haplotype. There was no difference in haplotype distribution between unknown mutations and normal controls. These results support a European origin for DeltaF508 alleles in Chilean patients, and make unlikely the presence of a predominant founder mutation in the so-far unknown alleles.     

Les génotypes responsables de mucoviscidose ou d’absence bilatérale des canaux déférents ABCD

Over the last decade, the genetic basis for CBAVD has been identified by its association with CFTR gene mutations, and CBAVD is now generally considered to be a mild or incomplete form of CF. In this review, the author summarizes the main results of compilation of CFTR gene analysis conducted in French laboratories for 3,923 patients with CF and 800 males with CABVD. The degree of clinical expression can be affected by several variables, including the molecular mechanisms by which the various CFTR mutations impair or disrupt the function of the CFTR chloride channel. Phenotypic expression of CFTR mutational genotypes varies from severe, progressive pulmonary disease with pancreatic insufficiency (CF-PI), to mild pulmonary disease with pancreatic sufficiency (PS) or singleorgan forms of “CFTR-opathies”. In CF, a total of 310 different CFTR mutations accounting for 94% of 7,846 CF alleles have generated almost 500 different genotypes, comprising 2 severe mutations in 88% of cases (CF-PI), one severe mutation in trans to a mild mutation in 11% (CF-PS), and 2 mild mutations in 1% of identified genotypes. In CBAVD, 137 mutations scattered over the whole gene were identified in 60% of 1,600 CBAVD alleles during the study. Among the 150 characterized mutational CFTR genotypes, compound heterozygosity was the rule, and the most frequent CBAVD combinations were ΔF508/5T (35%), ΔF508/other mutation (30%, including ΔF508/R117H-7T: 5,6%), and 5T/other mutation (17%). No combination of two severe mutations was found in CBAVD (0%); by contrast with the CF population, 88% of genotypes identified in CBAVD comprised a severe mutation in trans to a mild mutation, and 12% consisted of 2 mild mutations. A total of 22 genotypes were shared by both CF and CBAVD. The role of the 5T allele as a splicing variant with variable, incomplete disease penetrance in CBAVD is reviewed. Other haplotype backgrounds, such as the TG12 sequence and the M470V polymorphism, may influence CFTR splicing and/or function. This study confirms the high molecular heterogeneity of CFTR mutations in CBAVD and emphasizes the importance of extensive CFTR analysis in these patients. Longterm follow-up studies of CBAVD patients are necessary in order to predict the phenotypic consequences of numerous CFTR mutational genotypes.     

Geographic distribution and origin of CFTR mutations in Germany

The geographic distribution and origin of CFTR mutations in Germany was evaluated in 658 three-generation families with cystic fibrosis (CF). Fifty different mutations were detected on 1305 parental CF chromosomes from 22 European countries and overseas. The major mutation ΔF508 was identified on 71.5% of all CF chromosomes, followed by R553X (1.8%), N1303K (1.3%), G542X (1.1%), G551D (0.8%) and R347P (0.8%). According to the grandparents’ birthplace, 74% of CF chromosomes had their origin in Germany; the ΔF508 percentage was 77%, 75%, 70% and 62% in northern, southern, western and eastern Germany, respectively. Ten or more mutant alleles in the investigated CF gene pool originated from Austria, the Czech Republic, Poland, Russia, Turkey and the Ukraine. This widespread geographic origin of CFTR mutations in today’s Germany reflects the many demographic changes and migrations in Central Europe during the 20th century. Received: 10 October 1995 / Revised: 9 January 1995     

Extensive Sequencing of the CFTR gene: lessons learned from the first 157 patient samples

Cystic fibrosis (CF) is one of the most common monogenic diseases affecting Caucasians and has an incidence of approximately 1:3,300 births. Currently recommended screening panels for mutations in the responsible gene (CF transmembrane regulator gene, CFTR) do not detect all disease-associated mutations. Our laboratory offers extensive sequencing of the CFTR (ABCC7) gene (including the promoter, all exons and splice junction sites, and regions of selected introns) as a clinical test to detect mutations which are not found with conventional screening. The objective of this report is to summarize the findings of extensive CFTR sequencing from our first 157 consecutive patient samples. In most patients with classic CF symptoms (18/24, 75%), extensive CFTR sequencing confirmed the diagnosis by finding two disease-associated mutations. In contrast, only 5 of 75 (7%) patients with atypical CF had been identified with two CFTR mutations. A diagnosis of CF was confirmed in 10 of 17 (58%) newborns with either positive sweat chloride readings or positive immunoreactive trypsinogen (IRT) screen results. We ascertained ten novel sequence variants that are potentially disease-associated: two deletions (c.1641AG>T, c.2949_2853delTACTC), seven missense mutations (p.S158T, p.G451V, p.K481E, p.C491S, p.H949L, p.T1036N, p.F1099L), and one complex allele ([p.356_A357del; p.358I]). We ascertained three other apparently novel complex alleles. Finally, several patients were found to carry partial CFTR gene deletions. In summary, extensive CFTR gene sequencing can detect rare mutations which are not found with other screening and diagnostic tests, and can thus establish a definitive diagnosis in symptomatic patients with previously negative results. This enables carrier detection and prenatal diagnosis in additional family members.     

CFTR mutations in Turkish and North African cystic fibrosis patients in Europe: implications for screening

AIMS: To obtain more insight into the variability of the CFTR mutations found in immigrant cystic fibrosis (CF) patients who are living in Europe now, and to estimate the test sensitivity of different frequently used methods of DNA analysis to detect CF carriers or patients among these Turkish or North African immigrants. METHODS: A survey among 373 European CF centers asking which CFTR mutations had been found in Turkish and North African CF patients. RESULTS: 31 and 26 different mutations were reported in Turkish and North African patients, identifying 64.2% (113/176) and 87.4% (118/135) alleles, respectively (p < 0.001). The mean sensitivity (detection rate) of three most common CFTR mutation panels to detect these mutations differed between Turkish and North African people, 44.9% (79/176) versus 69.6% (94/135) (p < 0.001), and can be increased to 57.4% (101/176) and 79.3% (107/135) (p < 0.001), respectively, by expanding these panels with 13 mutations which have been found on two or more alleles. CONCLUSION: 35.8% and 12.6%, respectively, of CF alleles in Turkish and North African patients living in Europe now had not been identified. Among these populations, the test sensitivity of common CFTR mutation panels is insufficient for use in screening programs in Europe, even after expansion with frequent Turkish and North African mutations. This raises questions about whether and how to implement CF carrier and neonatal screening in a multiethnic society.     

Applicability of different antibodies for immunohistochemical localization of CFTR in sweat glands from healthy controls and from patients with cystic fibrosis.

The hereditary disease cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Understanding of the consequences of CFTR gene mutations is derived chiefly from in vitro studies on heterologous cell cultures and on cells hyperexpressing CFTR. Data from ex vivo studies on human tissue are scarce and contradictory, a fact which is in part explained by secondary tissue destruction in most affected organs. The purpose of this study was to establish conditions under which wild-type and mutated CFTR can be studied in affected human tissue. Sweat glands carry the basic defect underlying CF and are not affected by tissue destruction and inflammation. Therefore, we used this tissue to test a panel of eight different CFTR antibodies under various fixation techniques. The antibodies were tested on skin biopsy sections from healthy controls, from CF patients homozygous for the most common mutation, DeltaF508, and from patients carrying two nonsense mutations. Of the eight CFTR antibodies, only three-M3A7, MATG 1104, and cc24-met the criteria necessary for immunolocalization of CFTR in sweat glands. The labeling pattern in the CF sweat glands was consistent with the postulated processing defect of DeltaF508 CFTR. The antibodies exhibited different sensitivities for detecting DeltaF508 CFTR.     

A new mutation, 3905insT, accounts for 4.8% of 1173 CF chromosomes in Switzerland and causes a severe phenotype

We have analysed 1173 cystic fibrosis (CF) chromosomes from Switzerland for eight mutations in the CF transmembrane conductance regulator (CFTR) gene. This permitted the identification of 88.5% of all mutations present. A novel insertion mutation in exon 20 of the CFTR gene, 3905insT, was discovered. This mutation accounted for 4.8% of CFTR gene mutations in Switzerland and has since been identified in other populations of probable Swiss descent. It is associated with a highly variable clinical phenotype but always with pancreatic insufficiency. Haplotype analysis with three intragenic microsatellites in the CFTR gene showed that the mutation is associated with a haplotype rarely identified on other CFTR alleles and, therefore, that the frequency of the mutation in Switzerland is explained by a founder effect of a relatively recent mutation event. Received: 17 February 1997 / Accepted: 26 March 1977     

Genetic determination of exocrine pancreatic function in cystic fibrosis.

We showed elsewhere that the pancreatic function status of cystic fibrosis (CF) patients could be correlated to mutations in the CF transmembrane conductance regulator (CFTR) gene. Although the majority of CF mutations--including the most common, delta F508--strongly correlated with pancreatic insufficiency (PI), approximately 10% of the mutant alleles may confer pancreatic sufficiency (PS). To extend this observation, genomic DNA of 538 CF patients with well-documented pancreatic function status were analyzed for a series of known mutations in their CFTR genes. Only 20 of the 25 mutations tested were found in this population. They accounted for 84% of the CF chromosomes, with delta F508 being the most frequent (71%), and the other mutations accounted for less than 5% each. A total of 30 different, complete genotypes could be determined in 394 (73%) of the patients. The data showed that each genotype was associated only with PI or only with PS, but not with both. This result is thus consistent with the hypothesis that PI and PS in CF are predisposed by the genotype at the CFTR locus; the PS phenotype occurs in patients who have one or two mild CFTR mutations, such as R117H, R334W, R347P, A455E, and P574H, whereas the PI phenotype occurs in patients with two severe alleles, such as delta F508, delta I507, Q493X, G542X, R553X, W1282X, 621 + 1G----T, 1717-1G----A, 556delA, 3659delC, I148T, G480C, V520F, G551D, and R560T.     

XV-2c/KM19 haplotypes analysis of cystic fibrosis patients from western Mexico

The analysis of polymorphic markers within or closely linked to the cystic fibrosis transmembrane regulator (CFTR) gene is useful as a molecular tool for carrier detection of known and unknown mutations. To establish the association between mutations in the CFTR gene in western Mexican cystic fibrosis (CF) patients, the distribution of XV2c/KM19 haplotypes was analyzed by PCR and restriction enzyme digestion in 384 chromosomes from 74 CF patients, their unaffected parents, and normal subjects. The haplotype analysis revealed that haplotype B was present in 71.9% of CF chromosomes compared to 0% of non-CF chromosomes. The F508del and G542X mutations were strongly associated with haplotype B (96.7% and 100% of chromosomes, respectively). The haplotype distribution of the CF chromosomes carrying other CFTR mutations had a more heterogeneous background. Our results show that haplotype B is associated with CFTR mutations. Therefore, haplotype analysis is a suitable alternate strategy for screening CF patients with a heterogeneous clinical picture from populations with a high molecular heterogeneity where carrier detection programs are not available. In addition, it may be a helpful diagnostic tool for genetic counseling and carrier detection in the relatives of CF patients and in couples who are planning to have children.     

Spectrum of cystic fibrosis mutations in Serbia and Montenegro and strategy for prenatal diagnosis

We have screened 175 patients for molecular defects in the cystic fibrosis transmembrane conductance regulator (CFTR) gene using nondenaturing polyacrylamide gel electrophoresis (PAGE), denaturing gradient gel electrophoresis (DGGE), and sequencing. Six different mutations (F508del, G542X, 621+1G --> T, 2789+5G --> A, R1070Q, and S466X) accounted for 79.71% of CF alleles, with the F508del mutation showing a frequency of 72.28%. Another 12 mutations (R334W, 2184insA, I507del, 1525-1G --> A, E585X, R75X, M1I, 457TAT --> G, 574delA, 2723delTT, A120T, and 2907delTT) covered an additional 3.36%. A novel mutation (2723delTT) was found in one CF patient (F508del/2723delTT). Thus, a total of 18 mutations cover 82.57% of CF alleles. During our study, 72% of families at risk for having a CF child were found to be fully informative for prenatal diagnosis. Prenatal diagnosis was performed on 56 families; 76 analyses resulting in 16 affected, 38 carriers, and 22 healthy fetuses. These results imply that the molecular basis of CF in Serbia and Montenegro is highly heterogeneous, as is observed in other eastern and southern European populations. Because we detected more then 80% of CFTR alleles, results could be used for planning future screening and appropriate genetic counseling programs in our country.     

Maturation and function of cystic fibrosis transmembrane conductance regulator variants bearing mutations in putative nucleotide-binding domains 1 and 2.

One feature of the mutations thus far found to be associated with the disease cystic fibrosis (CF) is that many of them are clustered within the first nucleotide-binding domain (NBD) of the CF transmembrane conductance regulator (CFTR). We sought to discover the molecular basis for this clustering by introducing into the two NBDs of CFTR mutations either mimicking amino acid changes associated with CF or altering residues within highly conserved motifs. Synthesis and maturation of the mutant CFTR were studied by transient expression in COS cells. The ability of the altered proteins to generate cyclic AMP-stimulated anion efflux was assessed by using 6-methoxy-N-(sulfopropyl) quinolinium (SPQ) fluorescence measurements in HeLa cells expressing mutated plasmids. The results show that (i) all CF-associated mutants, with one exception, lack functional activity as measured in the SPQ assay, (ii) mutations in NBD1 are more sensitive to the effects of the same amino acid change than are the corresponding mutations in NBD2, (iii) cells transfected with plasmids bearing CF-associated mutations commonly but not exclusively lack mature CFTR, (iv) NBD mutants lacking mature CFTR fail to activate Cl- channels, and (v) the glycosylation of CFTR, per se, is not required for CFTR function. We reason that the structure of NBD1 itself or of the surrounding domains renders it particularly sensitive to mutational changes. As a result, most NBD1 mutants, but only a few NBD2 mutants, fail to mature or lack functional activity. These findings are consistent with the observed uneven distribution of CFTR missense mutations between NBD1 and NBD2 of CF patients.     

Increased frequency of CFTR gene mutations identified in Indian infertile men with non-CBAVD obstructive azoospermia and spermatogenic failure

High incidence of mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is associated with congenital bilateral absence of the vas deferens (CBAVD) and is considered as the genital form of cystic fibrosis (CF). The CFTR gene may also be involved in the etiology of male infertility in cases other than CBAVD. The present study was conducted to identify the spectrum and frequency of CFTR gene mutations in infertile Indian males with non-CBAVD obstructive azoospermia (n = 60) and spermatogenic failure (n = 150). Conspicuously higher frequency of heterozygote F508del mutation was detected in infertile males with non-CBAVD obstructive azoospermia (11.6%) and spermatogenic failure (7.3%). Homozygous IVS(8)-5T allele frequency was also significantly higher in both groups in comparison to those in normal healthy individuals. Two mutations in exon 25 viz., R1358I and K1351R were identified as novel mutations in patients with non-CBAVD obstructive azoospermia. Mutation R1358I was predicted as probably damaging CFTR mutation. This is the first report from the Indian population, emphasizing increased frequency of CFTR gene mutations in male infertility other than CBAVD. Thus, it is suggested that screening of CFTR gene mutations may be required in infertile Indian males with other forms of infertility apart from CBAVD and willing for assisted reproduction technology.     

Spectrum of CFTR mutations on Réunion Island: impact on neonatal screening

The large heterogeneity in the cystic fibrosis (CF) gene is the main difficulty for genotype characterization. Numerous studies have reported considerable variations in frequencies of CF transmembrane conductance regulator (CFTR) mutations in different populations, such as African, Asian, or European populations. To completely characterize the spectrum of mutations in the CFTR gene in the Réunion Island population, we screened 228 CF chromosomes using denaturing high-pressure liquid chromatography and denaturing gradient gel electrophoresis following by direct sequencing. We identified 27 mutations, accounting for 93% of CF chromosomes. They included three novel mutations (M1T, 3121-3C-->G, and L1324P), which are described in this paper. The detection of such a high proportion of Réunion Island CFTR mutations is important for improving neonatal screening of CF on Réunion Island.     

Analysis of four diverse population groups indicates that a subset of cystic fibrosis mutations occur in common among Caucasians.

To determine the nature and frequency of non-delta F508 cystic fibrosis (CF) mutations among diverse populations, we have sequenced exons 9-12 and 19-23 of the CF transmembrane conductance regulator (CFTR) gene from 128 CF chromosomes (39 U.S. Caucasian, 27 African-American, 42 Northern Irish, and 20 Israeli chromosomes). These regions were chosen because they encode the two putative ATP-binding folds of CFTR, domains which appear to have functional significance. In addition, CFTR exons 1 and 2 were analyzed in the American patients. Mutations were found on 49 of the 128 CF chromosomes. Nineteen different mutations were observed; six were novel, while the remaining 13 had been reported previously by our group or by other investigators. Six of nine different mutations found in African-American patients were unique to that population. However, the vast majority of the mutations found in U.S. Caucasians (eight of nine), Northern Irish (four of five), and Israelis (three of three) also occurred in other Caucasian groups. The preponderance of previously reported mutations in these three groups suggested that a subset of the non-delta F508 mutations occur in common among Caucasians. A survey of mutation frequencies in other Caucasian groups confirmed this observation. Unfortunately, this subset accounts for less than half of non-delta F508 CF mutations in most groups. These data suggest that screening for delta F508 and this select group of mutations will efficiently and economically maximize the number of CF mutations identified in Caucasian groups. However, it will be difficult to detect more than 90% of mutant CFTR alleles except in ethnically and geographically discrete populations where CF is the result of founder effect.