Glucose-6-phosphate dehydrogenase variants in Hawaii.

Sequence analysis has been performed on the DNA of 13 glucose-6-phosphate dehydrogenase (G6PD) deficient males from Hawaii, 6 of Filipino, 6 of Laotian, and 1 of Chinese extraction. Four different mutations were found: A-->T at cDNA nt 835, G-->A at nt 871, C-->T at nt 1360, and G-->A at nt 1388. The mutations at nt 835 and nt 1360 have not been described previously, and the latter, in particular, appears to be relatively common. The nt 1360 mutation changes the same codon as is altered in a previously described mutation, G6PD Andalus.     

High prevalence of SLC6A8 deficiency in X-linked mental retardation

A novel X-linked mental retardation (XLMR) syndrome was recently identified, resulting from creatine deficiency in the brain caused by mutations in the creatine transporter gene, SLC6A8. We have studied the prevalence of SLC6A8 mutations in a panel of 290 patients with nonsyndromic XLMR archived by the European XLMR Consortium. The full-length open reading frame and splice sites of the SLC6A8 gene were investigated by DNA sequence analysis. Six pathogenic mutations, of which five were novel, were identified in a total of 288 patients with XLMR, showing a prevalence of at least 2.1% (6/288). The novel pathogenic mutations are a nonsense mutation (p.Y317X) and four missense mutations. Three missense mutations (p.G87R, p.P390L, and p.P554L) were concluded to be pathogenic on the basis of conservation, segregation, chemical properties of the residues involved, as well as the absence of these and any other missense mutation in 276 controls. For the p.C337W mutation, additional material was available to biochemically prove (i.e., by increased urinary creatine : creatinine ratio) pathogenicity. In addition, we found nine novel polymorphisms (IVS1+26G-->A, IVS7+37G-->A, IVS7+87A-->G, IVS7-35G-->A, IVS12-3C-->T, IVS2+88G-->C, IVS9-36G-->A, IVS12-82G-->C, and p.Y498) that were present in the XLMR panel and/or in the control panel. Two missense variants (p.V629I and p.M560V) that were not highly conserved and were not associated with increased creatine : creatinine ratio, one translational silent variant (p.L472), and 10 intervening sequence variants or untranslated region variants (IVS6+9C-->T, IVS7-151_152delGA, IVS7-99C-->A, IVS8-35G-->A, IVS8+28C-->T, IVS10-18C-->T, IVS11+21G-->A, IVS12+15C-->T, *207G-->C, IVS12+32C-->A) were found only in the XLMR panel but should be considered as unclassified variants or as a polymorphism (p.M560V). Our data indicate that the frequency of SLC6A8 mutations in the XLMR population is close to that of CGG expansions in FMR1, the gene responsible for fragile-X syndrome.     

Molecular identification of mutations in G6PD gene in patients with favism in Iran

Glucose 6-phosphate dehydrogenase is a highly polymorphic enzyme encoded by a human X-linked gene (Xq2.8). This enzyme catalyses the first step of pentose phosphate pathway, that converts glucose 6-phosphate to 6-phosphogluconate with production of NADPH2. G6PD deficiency is the most common human metabolic inborn error affecting more than 400 million people world wide. The main clinical manifestations are acute hemolytic anemia and jaundice, triggered by infection or ingestion of Fava beans or oxidative drugs. A predominant variant of G6PD named Mediterranean is often associated with favism. This has been evident in several countries including Northern coastal provinces of Iran. Other current variants are Chatham and Cosenza. Molecular identification of the most prevalent mutations in G6PD gene was carried out in 71 males and females with G6PD deficiency. They were from Iranian Northern province of Golestan. DNA was extracted from blood samples and analyzed for known G6PD mutation by PCR and restriction fragment length polymorphisms (RFLP) technique. Adapting this method, revealed that Mediterranean mutation at nt 563(C-->T) is predominant in the area (69%) and 26.7% of patients have Chatham mutation at nt 1003(G-->A). Findings indicate a higher prevalence of these mutations, in Golestan compared to Mazandaran (66.2% Mediterranean and 19% Chatham mutation) and Gilan (86.4% Mediterranean and 9.71% Chatham mutations). Cosenza mutation at nt 1376(G-->C), by PCR-RFLP technique was not found among other 3 samples (4.3%). The similarity of these results with mutations in Italy indicates probable existence of a common ancestral origin in the observed populations.     

Detection of the most common G6PD gene mutations in Chinese using amplification refractory mutation system.

Glucose-6-phosphate dehydrogenase (G6PD) is the most common human enzymopathy. To date more than 122 mutations in the G6PD gene have been discovered, among which 12 point mutations are found in the Chinese. The 2 most common mutations, G1388A and G1376T, account for more than 50% of mutations representing various regions and ethnic groups in China. Setting up a simple and accurate method for detecting these mutations is not only useful for studying the frequency of the G6PD genotypes, but also for finding new mutations. The purpose of this study was to find a simple, inexpensive and accurate method for detecting these common mutations. The amplification refractory mutation system (ARMS) method was used in this study. Samples from 28 G6PD-deficient males were investigated. The natural and mismatched amplification and restriction enzyme digestion method was used as a standard method to evaluate the nature of the point mutations. Sixteen cases were found carrying the G1388A mutation and 12 the G1376T mutation. Fourteen cases of G1388A and 10 cases of G1376T were confirmed by ARMS. Four cases were not in concordance with the results obtained by the mismatched amplification-restriction enzyme digestion. These 4 cases were then judged by direct PCR sequencing at exon 12. The DNA sequencing data supported the results obtained by ARMS. Thus we concluded that the ARMS is a rapid, simple, inexpensive and accurate method for detecting the most common G6PD gene mutations among the Chinese.     

Autosomal recessive chronic granulomatous disease caused by novel mutations in NCF-2, the gene encoding the p67-phox component of phagocyte NADPH oxidase

Chronic granulomatous disease (CGD) is a rare inherited immunodeficiency disease that leads to severe recurrent infections. CGD is caused by defects in the phagocyte NADPH oxidase, a multiprotein enzyme that reduces oxygen to superoxide, a precursor of microbicidal oxidants. Less than 6% of CGD patients have an autosomal recessive form of the disease caused by mutations in NCF-2. This gene encodes p67-phox, a cytosolic oxidase subunit that associates with membrane-bound flavocytochrome b558 and regulates electron transfer. We studied six patients from five families with p67-phox deficiency and identified seven different mutant alleles. Patients from three of the kindreds were homozygous for their respective mutation, although the parents of only one family were known to be related. Five of the mutations have not previously been identified: (1) a missense mutation (383C-->T) in exon 5, (2) a nonsense mutation (196C-->T) in exon 3, (3) a missense mutation (230G-->A) in exon 3, (4) a nonsense mutation (298C-->T) in exon 4, and (5) a dinucleotide deletion (835-836 AC) from exon 9. Phagocytes from each of the patients analyzed failed to generate a measurable respiratory burst and had no detectable p67-phox protein. Our results further demonstrate that there is great heterogeneity among the mutations in p67-phox-deficient CGD patients, with no evidence for mutational hot-spots or a founder effect. Our data also support the hypothesis that the stability of p67-phox is particularly sensitive to missense mutations that cause amino acid substitutions within its N-terminal domain. In contrast, mutations predicting single amino acid changes elsewhere in the protein generally represent benign polymorphisms.     

Characterization of glucose-6-phosphate dehydrogenase deficiency and identification of a novel haplotype 487G>A/IVS5-612(G>C) in the Achang population of Southwestern China

The prevalence of glucose-6-phosphate dehydrogenase (G6PD) deficiency and its gene mutations were studied in the Achang population from Lianghe County in Southwestern China. We found that 7.31% (19 of 260) males and 4.35% (10 of 230) females had G6PD deficiency. The molecular analysis of G6PD gene exons 2–13 was performed by a PCR-DHPLC-Sequencing or PCR-Sequencing. Sixteen independent subjects with G6PD Mahidol (487G>A) and the new polymorphism IVS5-612 (G>C), which combined into a novel haplotype, were identified accounting for 84.2% (16/19). And 100% Achang G6PD Mahidol were linked to the IVS5-612 C. The percentage of G6PD Mahidol in the Achang group is close to that in the Myanmar population (91.3% 73/80), which implies that there are some gene flows between Achang and Myanmar populations. Interestingly, G6PD Canton (1376G>T) and G6PD Kaiping (1388G>A), which were the most common G6PD variants from other ethnic groups in China, were not found in this Achang group, suggesting that there are different G6PD mutation profiles in the Achang group and other ethnic groups in China. Our findings appear to be the first documented report on the G6PD genetics of the AChang people, which will provide important clues to the Achang ethnic group origin and will help prevention and treatment of malaria in this area.     

Mutagenesis of uracil-DNA glycosylase deficient mutants of the extremely thermophilic eubacterium Thermus thermophilus

Thermus thermophilus is an extremely thermophilic, aerobic, and gram-negative eubacterium that grows optimally at 70-75 degrees C, pH 7.5. In extremely high temperature environment, DNA damages in cells occur at a much higher frequency in thermophiles than mesophiles such as E. coli. When temperature rises, the deamination of cytosine residues in double-strand DNA is expected to increase greatly. T. thermophilus HB27 has two putative uracil-DNA glycosylase genes (udgA and udgB). Expression level of udgA gene was 2-3 times higher than that of udgB at 70, 74, and 78 degrees C when it was monitored by beta-glucosidase reporter assay. We developed hisD(3110), hisD(3113), hisD(3115), and hisD(174) marker allele that can specifically detect G:C-->A:T, C:G-->A:T, T:A-->A:T, and A:T-->G:C base-substitutions, respectively, by His(+) reverse mutations. We then disrupted udgA and udgB by thermostable kanamycin-resistant gene (htk) or pyrE gene insertion in each hisD background, and their spontaneous His(+) reversion frequencies were compared. A udgA,B double mutant showed a pronounced increase in G:C-->A:T reversion frequency compared with each single udg mutant, udgA or udgB. Estimated mutation rates of the udgA,B mutant cultured at 60, 70, and 78 degrees C were about 2, 12, and 117 His(+)/10(8)/generation, respectively. At 70 degrees C culture, increased ratio of the mutation rate compared with the udg(+) strain was 12-fold in udgA, 3-fold in udgB, and 56-fold in udgA,B mutant. On the other hand, no difference was observed in other mutations of C:G-->A:T, T:A-->A:T, and A:T-->G:C between udgA,B double mutant and the parent udg(+) strain. The present results indicated that gene products of udgB as well as udgA functioned in vivo to remove uracil in DNA and prevent G:C-->A:T transition mutations.     

Prevalence and Molecular Characterization of Glucose-6-Phosphate Dehydrogenase Deficiency at the China-Myanmar Border

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked hereditary disease that predisposes red blood cells to oxidative damage. G6PD deficiency is particularly prevalent in historically malaria-endemic areas. Use of primaquine for malaria treatment may result in severe hemolysis in G6PD deficient patients. In this study, we systematically evaluated the prevalence of G6PD deficiency in the Kachin (Jingpo) ethnic group along the China-Myanmar border and determined the underlying G6PD genotypes. We surveyed G6PD deficiency in 1770 adult individuals (671 males and 1099 females) of the Kachin ethnicity using a G6PD fluorescent spot test. The overall prevalence of G6PD deficiency in the study population was 29.6% (523/1770), among which 27.9% and 30.6% were males and females, respectively. From these G6PD deficient samples, 198 unrelated individuals (147 females and 51 males) were selected for genotyping at 11 known G6PD single nucleotide polymorphisms (SNPs) in Southeast Asia (ten in exons and one in intron 11) using a multiplex SNaPshot assay. Mutations with known association to a deficient phenotype were detected in 43.9% (87/198) of cases, intronic and synonymous mutations were detected alone in 34.8% (69/198) cases and no mutation were found in 21.2% (42/198) cases. Five non-synonymous mutations, Mahidol 487G>A, Kaiping 1388G>A, Canton 1376G>T, Chinese 4 392G>T, and Viangchan 871G>A were detected. Of the 87 cases with known deficient mutations, the Mahidol variant was the most common (89.7%; 78/87), followed by the Kaiping (8.0%; 7/87) and the Viangchan (2.2%; 2/87) variants. The Canton and Chinese 4 variants were found in 1.1% of these 87 cases. Among them, two females carried the Mahidol/Viangchan and Mahidol/Kaiping double mutations, respectively. Interestingly, the silent SNPs 1311C>T and IVS11nt93T>C both occurred in the same 95 subjects with frequencies at 56.4% and 23.5% in tested females and males, respectively (P<0.05). It is noteworthy that 24 subjects carrying the Mahidol mutation and two carrying the Kaiping mutation also carried the 1311C>T/IVS11nt93T>C SNPs. Further studies are needed to determine the enzyme levels of the G6PD deficient people and presence of additional G6PD mutations in the study population.     

Structure of the SLC7A7 gene and mutational analysis of patients affected by lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive defect of cationic amino acid transport caused by mutations in the SLC7A7 gene. We report the genomic structure of the gene and the results of the mutational analysis in Italian, Tunisian, and Japanese patients. The SLC7A7 gene consists of 10 exons; sequences of all of the exon-intron boundaries are reported here. All of the mutant alleles were characterized and eight novel mutations were detected, including two missense mutations, 242A-->C (M1L) and 1399C-->A (S386R); a nonsense mutation 967G-->A (W242X); two splice mutations IVS3 +1G-->A and IVS6 +1G-->T; a single-base insertion, 786insT; and two 4-bp deletions, 455delCTCT and 1425delTTCT. In addition, a previously reported mutation, 1625insATCA, was found in one patient. It is noteworthy that 242A-->C causes the change of Met1 to Leu, a rare mutational event previously found in a few inherited conditions. We failed to establish a genotype/phenotype correlation. In fact, both intrafamilial and interfamilial phenotypic variability were observed in homozygotes for the same mutation. The DNA-based tests are now easily accessible for molecular diagnosis, genetic counseling, and prenatal diagnosis of LPI.     

Methylation and repeats in silent and nonsense mutations of p53

All exonic CG sequences in p53 are methylated; this epigenetic modification is correlated with frequent G:C-->A:T transitions in p53. Recent reports reveal the presence in p53 of non-CG methylation in CC and CCC sequences, complementary to sites of selective guanosine adduct formation (GG and GGG), and the association of genetic instability with methylation at repetitive sequences. We presently investigated the distribution of methylation sites and repetitive elements in silent and nonsense p53 mutations (2051) among the IARC's TP53 somatic mutation database for exons 5-8. Silent mutations are nonrandom, but mostly involve G:C-->A:T transitions (62%); in particular C-->T mutations (39% of all silent mutations) are mostly correlated with CC and CCC sequences, while G-->A mutations with GG sequences. Sequence analysis of all non-G:C-->A:T silent mutations reveals the frequent formation of new methylation sites (CG), new CCC and GGG sequences in the resulting sequence, refinement of symmetry elements at interrupted microsatellite-like sequences and formation of small repeats (55.3%). The G:C-->A:T silent mutations characterize cancers associated with cigarette smoking (e.g. bladder or lung and bronchus cancer versus colorectal cancer); on the contrary, non-G:C-->A:T silent mutations have similar frequencies in most cancers. Nonsense mutations in exons 5-8, all resulting in mutants lacking amino acids 307-393, which are crucial for p53 activity, were also analyzed. The frequency of nonsense mutations is higher at methylated sites or repeats 1-2 nucleotides removed from methylation sites. Frameshift mutations are also more frequent at repeated sequences. The frequent G:C-->A:T silent mutations could indicate that CC and CCC sequences of exons 5-8 are occasionally targets of non-CpG methylation of cytosine. This process of de novo methylation in the presence of microsatellite-like sequences and small repeats might influence the genetic stability of a variety of genes.     

Characterization of glucose-6-phosphate dehydrogenase deficiency and identification of a novel haplotype 487G>A/IVS5-612(G>C) in the Achang population of southwestern China

The prevalence of glucose-6-phosphate dehydrogenase (G6PD) deficiency and its gene mutations were studied in the Achang population from Lianghe County in Southwestern China. We found that 7.31% (19 of 260) males and 4.35% (10 of 230) females had G6PD deficiency. The molecular analysis of G6PD gene exons 2―13 was performed by a PCR-DHPLC-Sequencing or PCR-Sequencing. Sixteen inde-pendent subjects with G6PD Mahidol (487G>A) and the new polymorphism IVS5-612 (G>C), which combined into a novel haplotype, were identified accounting for 84.2% (16/19). And 100% Achang G6PD Mahidol were linked to the IVS5-612 C. The percentage of G6PD Mahidol in the Achang group is close to that in the Myanmar population (91.3% 73/80), which implies that there are some gene flows between Achang and Myanmar populations. Interestingly, G6PD Canton (1376G>T) and G6PD Kaiping (1388G>A), which were the most common G6PD variants from other ethnic groups in China, were not found in this Achang group, suggesting that there are different G6PD mutation profiles in the Achang group and other ethnic groups in China. Our findings appear to be the first documented report on the G6PD genetics of the AChang people, which will provide important clues to the Achang ethnic group origin and will help prevention and treatment of malaria in this area.     

Genetic basis of inosine triphosphate pyrophosphohydrolase deficiency

Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is a common inherited condition characterized by the abnormal accumulation of inosine triphosphate (ITP) in erythrocytes. The genetic basis and pathological consequences of ITPase deficiency are unknown. We have characterized the genomic structure of the ITPA gene, showing that it has eight exons. Five single nucleotide polymorphisms were identified, three silent (138G-->A, 561G-->A, 708G-->A) and two associated with ITPase deficiency (94C-->A, IVS2+21A-->C). Homozygotes for the 94C-->A missense mutation (Pro32 to Thr) had zero erythrocyte ITPase activity, whereas 94C-->A heterozygotes averaged 22.5% of the control mean, a level of activity consistent with impaired subunit association of a dimeric enzyme. ITPase activity of IVS2+21A-->C homozygotes averaged 60% of the control mean. In order to explore further the relationship between mutations and enzyme activity, we examined the association between genotype and ITPase activity in 100 healthy controls. Ten subjects were heterozygous for 94C-->A (allele frequency: 0.06), 24 were heterozygotes for IVS2+21A-->C (allele frequency: 0.13) and two were compound heterozygous for these mutations. The activities of IVS2+21A-->C heterozygotes and 94C-->A/IVS2+21A-->C compound heterozygotes were 60% and 10%, respectively, of the normal control mean, suggesting that the intron mutation affects enzyme activity. In all cases when ITPase activity was below the normal range, one or both mutations were found. The ITPA genotype did not correspond to any identifiable red cell phenotype. A possible relationship between ITPase deficiency and increased drug toxicity of purine analogue drugs is proposed.     

The facile detection of 1505G-->A in Gaucher patients with different phenotypes

In Gaucher disease patients, over 100 disease-causing mutations have been identified. For identification of the 1504C-->T (R463C) mutation it is common to use PCR-restriction fragmentation analysis using the restriction enzyme MspI. In the present study we investigated the reliability of this approach because accurate determination of genotypes is important in genotype-phenotype correlations. A simple modification, i.e. using the restriction enzyme HphI instead of MspI, revealed that type I and II Gaucher disease patients who had previously been identified as carrying the 1504C-->T mutation in fact carried the 1505G-->A (IVS10(-1)G-->A) mutation. Sequencing of the appropriate fragment confirmed this. The PCR method easily differentiates between these two mutations in Gaucher disease patients, thus circumventing the need for sequencing procedures. The phenotypes of the patients found to be carrying the 1505G-->A mutation are also described.     

Mutations in PIP5K3 are associated with François-Neetens mouchetée fleck corneal dystrophy

Fran?ois-Neetens fleck corneal dystrophy (CFD) is a rare, autosomal dominant corneal dystrophy characterized by numerous small white flecks scattered in all layers of the stroma. Linkage analysis localized CFD to a 24-cM (18-Mb) interval of chromosome 2q35 flanked by D2S2289 and D2S126 and containing PIP5K3. PIP5K3 is a member of the phosphoinositide 3-kinase family and regulates the sorting and traffic of peripheral endosomes that contain lysosomally directed fluid phase cargo, by controlling the morphogenesis and function of multivesicular bodies. Sequencing analysis disclosed missense, frameshift, and/or protein-truncating mutations in 8 of 10 families with CFD that were studied, including 2256delA, 2274delCT, 2709C-->T (R851X), 3120C-->T (Q988X), IVS19-1G-->C, 3246G-->T (E1030X), 3270C-->T (R1038X), and 3466A-->G (K1103R). The histological and clinical characteristics of patients with CFD are consistent with biochemical studies of PIP5K3 that indicate a role in endosomal sorting.     

Characterization of missense mutations and large deletions in the ALPL gene by sequencing and quantitative multiplex PCR of short fragments

Hypophosphatasia is a rare inherited bone disorder characterized by defective bone and dental mineralization and deficiency of serum and liver/bone/kidney alkaline phosphatase activity. The disease is due to mutations in the alkaline phosphatase liver-type (ALPL) gene. Gross deletions or insertions have not previously been reported in this gene. We report here the characterization of nine novel ALPL gene mutations in a series of 8 patients affected by various forms of hypophosphatasia. The newly discovered mutations included five missense mutations (c.368C --> A, c.814C--> T, c.1196C--> T, c.1199C--> T, c.1283G--> C), two small deletions (c.797_802del, c.1044_1055del), and two large deletions. The large deletions were detected by quantitative multiplex polymerase chain reaction (PCR) of short fluorescent fragments (QMPSF). We conclude that QMPSF slightly reduces the proportion of undetected mutations in hypophosphatasia and improves genetic counselling in the affected families.     

The prevalence of factor V Leiden,prothrombin G20210A and methylenetetrahydrofolate reductase polymorphism C677T among G6PD deficient individuals from Western Iran

It has been suggested that the allele frequency of thrombophilic mutations is affected by glucose-6-phosphate dehydrogenase (G6PD) deficiency. The prevalence of thrombophilic mutations were studied in sixty G6PD deficient individuals including 57 males and three females with the mean age of 15 ± 3.08 and 110 age and sex matched healthy individuals consisted of 95 males and 15 females with the mean age of 16.19 ± 2.17 from the Kermanshah Province of Iran. Using a combination of PCR-RFLP technique, single strand conformation polymorphism (SSCP) analysis and DNA sequencing polymorphic G6PD mutations were identified. The factor V Leiden, prothrombin G20210A and methylenetetrahydrofolate reductase (MTHFR) C677T were detected by PCR-RFLP method using MnlI, HindIII and HinfI restriction enzymes, respectively. Three mutations, G6PD Mediterranean, G6PD Chatham and G6PD Cosenza were identified in 60 G6PD deficient individuals with highest prevalence of G6PD Mediterranean (91.6%). In G6PD deficient individuals the prevalence of factor V Leiden tended to be higher (5%) compared to healthy individuals (2.7%). The prevalence of prothrombin G20210A mutation in G6PD deficient individuals was 1.7%. However, in normal subjects the prevalence of this mutation was 2.7%. The frequency of T allele in G6PD deficient individuals were insignificantly higher (29.16%) than those in healthy individuals (26.8%). Our finding indicates that the prevalence of factor V Leiden, prothrombin G20210A and MTHFR C677T in G6PD deficient individuals is not statistically different compared to normal subjects and G6PD deficiency is not associated with these thrombophilic mutations in Western Iran.     

The molecular basis of Sjögren-Larsson syndrome: mutation analysis of the fatty aldehyde dehydrogenase gene

Sj?gren-Larsson syndrome (SLS) is an autosomal recessive disorder characterized by ichthyosis, mental retardation, spasticity, and deficient activity of fatty aldehyde dehydrogenase (FALDH). To define the molecular defects causing SLS, we performed mutation analysis of the FALDH gene in probands from 63 kindreds with SLS. Among these patients, 49 different mutations-including 10 deletions, 2 insertions, 22 amino acid substitutions, 3 nonsense mutations, 9 splice-site defects, and 3 complex mutations-were found. All of the patients with SLS were found to carry mutations. Nineteen of the missense mutations resulted in a severe reduction of FALDH enzyme catalytic activity when expressed in mammalian cells, but one mutation (798G-->C [K266N]) seemed to have a greater effect on mRNA stability. The splice-site mutations led to exon skipping or utilization of cryptic acceptor-splice sites. Thirty-seven mutations were private, and 12 mutations were seen in two or more probands of European or Middle Eastern descent. Four single-nucleotide polymorphisms (SNPs) were found in the FALDH gene. At least four of the common mutations (551C-->T, 682C-->T, 733G-->A, and 798+1delG) were associated with multiple SNP haplotypes, suggesting that these mutations originated independently on more than one occasion or were ancient SLS genes that had undergone intragenic recombination. Our results demonstrate that SLS is caused by a strikingly heterogeneous group of mutations in the FALDH gene and provide a framework for understanding the genetic basis of SLS and the development of DNA-based diagnostic tests.