Dominant influence of gamma-globin promoter polymorphisms on fetal haemoglobin expression in sickle cell disease.

Polymorphisms of multiple cis-acting elements in the beta-globin locus are associated with variable fetal haemoglobin (HbF) level in sickle cell disease. We developed a multiplex assay permitting simultaneous analysis of three polymorphic cis elements spanning 53 kb of the beta-globin locus. We identified concordance between polymorphic alleles in gamma- and beta-globin promoters however a significant number of betaS-chromosomes were identified with polymorphisms in hypersensitive site 2 (HS2) of the beta-globin locus control region juxtaposed to atypical cis alleles in the gamma-promoter. Analysis of an unusually large number of such hybrid haplotype chromosomes provided unique insight into HbF level associated with specific cis alleles. Associations between cis alleles and HbF level in patients were verified by in vitro functional analysis. Our findings indicate that compared to HS2, polymorphism in the gamma-promoter exerts a dominant influence on HbF level in sickle cell disease.

Rapid identification of hemoglobin variants by electrospray ionization mass spectrometry.

The precise identification of human hemoglobin variants, over 700 human hemoglobin variants are known, is essential for prediction of their clinical and genetic significance. A systematic approach to their rapid identification is described. Traditionally this requires protein or DNA characterization which entails lengthy analytical procedures. To overcome these obstacles a rapid approach to variant hemoglobin identification has been developed using conventional phenotypic methods combined with electrospray ionization-mass spectrometry (ESI-MS). The latter requires only a small amount of whole blood (10 microl) but in most cases 2 microl would have been sufficient and no preanalytical steps, such as separation of red cells or globin chains, are necessary. Aged, hemolyzed blood samples can also be analyzed. This approach has been used to positively identify 95% of the variants in over 250 samples. The remaining 5% in which a variant was detected by phenotypic techniques were not resolved by mass spectrometry. Ninety-nine different abnormalities comprising 36 alpha-chain variants, 59 beta-chain variants (including 2 extensions), and 4 hybrid hemoglobins were identified. These include 15 novel variants. The application of ESI-MS described requires approximately 1 h to prepare and analyze each sample and has minimal reagent costs. The turnaround time on a single sample can be as little as 2 h. This technique can now be considered a useful additional tool for reference laboratories.

Nucleotide variation regulates the level of enhancement by hypersensitive site 2 of the beta-globin locus control region.

The beta-globin locus control region hypersensitive site 2 (HS2) enhancer possesses a unique property for stimulating high-level globin gene expression. Although the deletion of cis-acting motifs influences the level of enhancement conferred by HS2, there is controversy on whether polymorphism of the same elements contributes to variation of the fetal hemoglobin (HbF) level among patients with sickle cell anemia. We analyzed reporter gene activity of constructs containing variant HS2 enhancers derived from beta(S) chromosomes to directly test the effect of polymorphism on enhancer activity. Constructs containing four enhancer variants linked to an identical gamma-globin promoter showed markedly different levels of reporter gene activity. Juxtaposition of HS2 derived from the Asian and Senegal chromosomes, which are associated with similarly high levels of HbF, to cognate sequence extending to -1500 of the (G)gamma globin gene showed significantly different levels of reporter gene activity. Our findings indicate that nucleotide variation regulates the level of enhancement conferred by HS2; however, the reporter activities showed no correlation with the level of Hb F associated with the common beta(S) chromosomes.

Factor VIII gene polymorphisms in the Asian Indian population.

Little is known about the heterozygous frequency of factor VIII gene markers in the Asian Indian population. The objective of this study was to establish the heterozygous frequency of polymorphic markers within and flanking the factor VIII gene in Indians and identify those most informative for carrier screening and prenatal diagnosis. Factor VIII gene polymorphism analysis at intragenic and extragenic sites was carried out by the polymerase chain reaction (PCR) method and Southern blot procedure. Sixty-three Asian Indian haemophiliacs and their families were screened. A control group of 150 women from nonhaemophilic families were screened for two markers, HindIII and BclI. Among the intragenic markers studied, the HindIII restriction fragment length polymorphism (RFLP) showed the highest heterozygous frequency (0.52) followed by the intron 13 (0.47) and intron 22 (0. 44) short tandem repeats (STRs). Among extragenic markers, TaqI had the highest heterozygous frequency (0.75) followed by BglII (0.54). The intron 22 inversion mutation was observed in eight (40%) of 20 severe cases. In the population studied the most diagnostic polymorphisms were the intragenic markers, intron 22 (70%) STR followed by the intron 13 (52%) STR and HindIII (52%) RFLP, and the TaqI (50%) extragenic marker. Application of HindIII, BclI and the intron 22 dinucleotide repeat combined were diagnostic in 87.2% of haemophilia A families studied.

Reversal of metabolic block in glycolysis by enzyme replacement in triosephosphate isomerase-deficient cells.

Inherited deficiency of the housekeeping enzyme triosephosphate isomerase (TPI) is the most severe clinical disorder of glycolysis. Homozygotes manifest congenital hemolytic anemia and progressive neuromuscular impairment, which in most cases pursues an inexorable course with fatal outcome in early childhood. No effective therapy is available. Hitherto specific enzyme replacement has not been attempted in disorders of glycolysis. Primary skeletal muscle myoblasts and Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines generated from homozygous TPI-deficient patients were cultured in the presence of exogenous enzyme or cocultured with human K562 erythroleukemia cells as an exogenous source of TPI. Uptake of active enzyme by TPI-deficient cells resulted in reversal of intracellular substrate accumulation, with a reduction in dihydroxyacetone phosphate (DHAP) concentration to levels seen in TPI-competent cells. Evidence of successful metabolic correction of TPI deficiency in vitro establishes the feasibility of enzyme replacement therapy, and has important implications for the potential role of allogeneic bone marrow transplantation and gene therapy as a means of sustained delivery of functional enzyme in vivo.

Unique and recurrent WAS gene mutations in Wiskott-Aldrich syndrome and X-linked thrombocytopenia.

Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT) are allelic phenotypes caused by defects of the WAS gene. Fourteen distinct mutations including seven novel gene defects in 16 WAS and four XLT patients were identified by single strand conformation polymorphism analysis and DNA sequencing of the WAS gene. Eleven (79%) of these mutations are located within exons 1 to 4 with clustering in exon 2. Carrier detection in 33 at-risk females and prenatal diagnosis at 12 weeks gestation in one family with a novel WAS mutation was performed by direct mutation analysis. A remarkably high frequency (72%) of point mutations involved CpG dinucleotides. C-->T or G-->A transitions at CpG sites were identified in all isolated WAS cases (n = 7). Allele frequencies for the dinucleotide repeat at locus DXS6940 were determined in Northern European, African and Asian populations. Mutation screening alone or in combination with analysis of polymorphic loci DXS6940 and DXS255 delineated the germline origin of a unique insertion mutation and four recurrent CpG mutations, three of which arose spontaneously during maternal gametogenesis.

Ancestral origin of variation in the triosephosphate isomerase gene promoter.

A high frequency of nucleotide substitutions -5A/G, -8G/A, -24T/G in the triosephosphate isomerase (TPI) gene promoter has been demonstrated in African-Americans. The biological significance of these promoter variants, two of which, -8G/A and -24T/G, occur within regulatory elements essential for transcription, is controversial. The geographical distribution and frequency of allelic variation in the TPI promoter was determined in 378 unrelated normal subjects from sub-Saharan African (n = 103), Caribbean (n = 26), Northern European (n = 57), Mediterranean (n = 55), Middle Eastern (n = 42), Asian Indian (n = 48) and Oriental (n = 47) populations. Five haplotypes were identified: the common haplotype, -5A-8G-24T, -5G, -8A, -5G-8A, and -5G-8A-24G. All, with the exception of the -8A haplotype, were present in geographically dispersed populations. The -5G allele, which was found at varying frequency in all groups, has attained high frequency in the African, Caribbean and Oriental populations. Phylogenetic comparison suggests this may represent the ancestral promoter haplotype. Homozygosity for the -5G-8A haplotype identified in four subjects confirms that these variants are not responsible for a null allele as formerly postulated. Linkage disequilibrium between related TPI promoter haplotypes, -5G, -5G-8A and -5G-8A-24G, and a single nucleotide polymorphism at nt2262 of the TPI gene supports a single ancestral origin for these mutations which precedes the separation of African and non-African populations.

Ancestral origin of variation in the triosephosphate isomerase gene promoter

A high frequency of nucleotide substitutions -5A/G, -8G/A, -24T/G in the triosephosphate isomerase (TPI) gene promoter has been demonstrated in African-Americans. The biological significance of these promoter variants, two of which, -8G/A and -24T/G, occur within regulatory elements essential for transcription, is controversial. The geographical distribution and frequency of allelic variation in the TPI promoter was determined in 378 unrelated normal subjects from Sub-Saharan African (n = 103), Caribbean (n = 26), Northern European (n = 57), Mediterranean (n = 55), Middle Eastern (n = 42), Asian Indian (n = 48) and Oriental (n = 47) populations. Five haplotypes were identified: the common haplotype, -5A-8G-24T, -5G, -8A, -5G-8A, and -5G-8A-24G. All, with the exception of the -8A haplotype, were present in geographically dispersed populations. The -5G allele, which was found at varying frequency in the African, Caribbean and Oriental populations. Phylogenetic comparison suggests this may represent the ancestral promoter haplotype. Homozygosity for the -5G-8A haplotype identified in four subjects confirms that these variants are not responsible for a null allele as formerly postulated. Linkage disequilibrium between related TPI promoter haplotypes, -5G, -5G-8A and -5G-8A-24G, and a single nucleotide polymorphism at nt2262 of the TPI gene supports a single ancestral origin for these mutations which preceeds the separation of African populations.(Au)

Localization of a gene for familial hemophagocytic lymphohistiocytosis at chromosome 9q21.3-22 by homozygosity mapping.

Familial hemophagocytic lymphohistiocytosis (FHL), also known as familial erythrophagocytic lymphohistiocytosis and familial histiocytic reticulosis, is a rare autosomal recessive disorder of early childhood characterized by excessive immune activation. Linkage of the disease gene to an approximately 7.8-cM region between markers D9S1867 and D9S1790 at 9q21.3-22 was identified by homozygosity mapping in four inbred FHL families of Pakistani descent with a combined maximum multipoint LOD score of 6.05. This is the first genetic locus to be described in FHL. However, homozygosity by descent across this interval could not be demonstrated in an additional affected kindred of Arab origin, whose maximum multipoint LOD score was -0.12. The combined sample revealed significant evidence for linkage to 9q markers (LOD score with heterogeneity, 5.00). Identification of the gene(s) involved in the pathogenesis of FHL will contribute to an understanding of the control of T-lymphocyte and macrophage activation, which is central to homeostasis in the immune system.

Localisation of cis regulatory elements at the beta-globin locus: analysis of hybrid haplotype chromosomes.

Several cis elements at the beta-globin gene cluster and the upstream locus control region (LCR) have been implicated in modulation of fetal haemoglobin (Hb F) level in beta-globin disorders. To determine the role of elements at the LCR and the beta-globin gene cluster on HbF level among sickle cell anaemia (SCA) patients, hybrid haplotype betaS chromosomes exhibiting variation in the association of alleles of LCR hypersensitive site 2 (HS2) and the beta-globin gene cluster restriction fragment length polymorphosim (RFLP) haplotypes were identified in an unselected population of 100 patients. On 15 chromosomes the polymorphic HS2 short tandem repeat(TA)xN10-12(TA)y containing a Hox2 binding motif differed from that typically associated with the corresponding beta-globin gene cluster RFLP haplotype. Among patients homozygous for the Benin RFLP haplotype, in whom one chromosome carried the (TA)9N10(TA)10 allele, no effect on HbF level was observed. Polymorphism of the pre-Ggamma framework, an enhancer located 25 kb downstream of HS2 localised the breakpoint for each of these 'hybrid' haplotype chromosomes upstream of this element. Previously described hybrid haplotype chromosomes with the (TA)9N10(TA)10 HS2 allele associated with raised HbF by contrast arise by recombination 1 kb downstream of the pre-Ggamma framework. This study suggests that variability in HbF level associated with polymorphisn of the HS2 enhancer depend on downstream determinant (s) in tight linkage disequilibrium with HS2. The pre-Ggamma framework is the only known polymorphic cis-active determinant in this region.

Quantification of Ggamma- and Agamma-globins by electrospray ionisation mass spectrometry.

Elucidation of the molecular basis for persistent fetal haemoglobin (Hb F) production in adult life has important implications for the pathophysiology and treatment of human beta haemoglobinopathies. Electrospray ionisation mass spectrometry (ESMS) was applied to analyse the pattern of gamma-globin expression in patients with hereditary persistence of fetal haemoglobin (HPFH) and sickle cell anaemia (SCA). Ggamma and Agamma-globin chains were identified by their measured molecular masses and distinguished by mass difference (14 Da) following deconvolution of ESMS spectra using maximum entropy based software. Prediction of HPFH type by ESMS was confirmed by molecular analysis. Direct determination of Ggamma:Agamma globin chain ratio from whole blood by the novel application of ESMS provides a rapid and sensitive approach to characterisation of gamma-globins and facilitates correlation of gamma-globin level and polymorphism of cis-active elements at the beta-globin locus.

Interethnic variation in genetic risk factors for vascular disease

Genetic predisposition to vascular disease has important implications for population screening and prevention. The most common hereditary cause of venous thrombosis is resistance to activated protein C caused by the G1691A point mutation in exon 10 of the factor V gene (1q21-25) which leads to the substitution of glutamine for arginine (factor V Leiden). A thermolabile variant of 5, 10-methylenetetrahydrofolate reductase (MTHFR) caused by the C677T mutation of the MTHFR gene (1p36.3) which substitutes valine for alanine is associated with the vascular disease risk factor hyperhomocysteinaemia. The possibility that these mutations may predispose individuals of African-American origin to thrombosis was investigated in 9 patients (6 male, 3 female) with sickle cell anaemia who had experienced a thrombotic episode. The frequency of the MTHFR C677T mutation was also determined in unrelated subjects from six different populations: African-Caribbean (50), Oriental (47), Asian Indian (21), Middle Eastern (24), Meditteranean (50) and Northen European (61). The MTHFR and factor gene regions of interest were amplified by the polymerase chain reaction method. Factor V Leiden was screened for by single strand conformation polymorphism analysis and the MTHFR C 677T mutation by Hinf 1 restriction. All patients were homozygous normal (G/G) for the factor V allele. This is consistent with population studies which failed to identify factor V Leiden in normal subjects of Sub-Saharan African populations and found a low frequency (0.65 percent in Black Americans. By contrasts, factor V Leiden was found to be most prevalent in European populations (from 1.4 percent in Finland to 7 percent in Greece). One patient was heterozygous (C/T) and 8 homozygous normal (C/C) for the MTHFR mutation. Population studies revealed the observed frequency of the mutant allele (T) to be lowest in African-Caribbean subjects (9 percent) of whom none were homozygous and only 18 percent heterozygous. The frequency was highest in the Meditteranean population (42 percent), followed by Middle Eastern (38 percent), Northern European (30 percent), Asian Indian (21 percent) and Oriental (19 percent). No deviation from Hardy-Weinberg equilibrium was detected. The proportion of subjects homozygous for the mutation (T/T) was 18 percent Meditteranean, 17 percent Middle East, 10 percent Northern European and Asian Indian and 2 percent Oriental. (AU)

The molecular basis for genetic variation in fetal haemoglobin level in sickle cell anaemia

Raised fetal haemoglobin levels ameliorate the clinical severity of sickle cell anaemia. This provides a rationale for therapy and signals the need to elucidate the molecular basis for the variability of HbF level in sickle cell anaemia. Polymorphism within regulatory sites of the globin locus alter the affinity with which transcription factors bind their cogante recognition sites thereby modulating gene expression. A novel chromosomal haplotype utilising polymorphic variation within two enhancers hypersensitive site 2 (HS2) of the locus control region and the pre g y framework and the silencer protein BP1 binding site that spans a 53 kb interval of the globin locus was determined in 205 patients with sickle cell anaemia from the UK and Jamaica. Multiplexed polymerase chain reactions developed to facilitate rapid analysis of polymorphisms within each site allowed individual haplotype construction in a single lane of a single strand conformation polymorphism (SSCP) electrophoresis gel. SSCP banding patterns for the combined polymorphic sites were confirmed as unique chromosomal haplotypes by DNA sequence anaylsis. Three hundred and ten chromosomes with sequence TA7 N 12 TA8, GA and AT(AT)8T4 were designated class 1. Twenty-five class II with sequence TA8 N10 TA11, GG, AC(AT)6T9; 17 class III with TA9 N10 TA10, AG, AC(AT)8T4; 7 class IV with TA10 N10 TA12, AG, AC(AT)9T5 and 13 class Ia haplotype with sequence TA9 N10 TA10, GA, AT(AT)8T4 were identified. The proportion of class I chromosomes in both groups (159/210 UK; 151/200 Jamaican) is identical, however, significantly more chromosomes with sub-classes I and II are present among the Jamaican sample. There is incomplete association between the functional haplotype classes defined and conventional haplotypes based on restriction fragment length polymorphisms. The level of Hbf is significantly higher in patients with functional haplotype classes III and IV compared to those with classes I and II. Both high HbF haplotypes share a high affinity binding motif for the transcription factor GATA-1. This novel approach allows the combined effets of genetic variation in regulatory sequences within the globin locus on HbF level to be defined. (AU)

Evidence for founder effect of the Glu104Asp substitution and identification of new mutations in triosephosphate isomerase deficiency.

Triosephosphate isomerase (TPI) deficiency is an autosomal recessive disorder of glycolysis characterized by multisystem disease and lethality in early childhood. Among seven unrelated Northern European kindreds with clinical TPI deficiency studied, a single missense mutation at codon 104 (GAG;Glu-->GAC;Asp) predominated, accounting for 11/14 (79%) mutant alleles. In three families molecular analysis revealed compound heterozygosity for Glu104Asp and novel missense mutations. In two cases the second mutation was a Cys to Tyr substitution at codon 41 (TGT-->TAT) and in one an Ile to Val substitution at codon 170(ATT-->GTT). The origin of the Glu104Asp mutation was defined by haplotype analysis using a novel G/A polymorphism at nucleotide 2898 of the TPI gene. Cosegregation of the low frequency 2898A allele with the G-->C base change at nucleotide 315 supports a single origin for the Glu104Asp mutation in a common ancestor.

Predominance of the HLA-H Cys282Tyr mutation in Austrian patients with genetic haemochromatosis.

BACKGROUND/AIMS: Genetic haemochromatosis is the most common autosomal recessive disorder in Northern European populations. A major histocompatibility complex class I-like gene, HLA-H, has been proposed to be responsible for genetic haemochromatosis. The prevalence of HLA-H gene mutations 282(TGC; Cys/TAC; Tyr) and 63(CAT; His/GAT; Asp) was determined in patients of Austrian origin. METHODS: DNA extracted from the blood of 40 Austrian patients and 271 controls was used to amplify HLA-H gene fragments by the polymerase chain reaction method. The base changes responsible for mutations Cys282Tyr and His63Asp alter recognition sites for restriction enzymes SnaB I and Bcl I, respectively. Digestion products were separated by agarose gel electrophoresis and visualised by ethidium bromide staining. RESULTS: Thirty-one (77.5%) genetic haemochromatosis patients were homozygous for mutation Cys282Tyr and three compound heterozygous for mutations Cys282Tyr and His63Asp. One patient was homozygous for mutation His63Asp but normal for mutation Cys282Tyr. Four patients were normal at both genetic loci and one patient was heterozygous for mutation His63Asp. One control subject homozygous for mutation Cys282Tyr was found on investigation to fulfill diagnostic criteria for haemochromatosis. Eight control subjects homozygous for mutation His63Asp showed no biochemical or clinical evidence of haemochromatosis indicating that this variant is not directly responsible for haemochromatosis. Absence of the Cys282Tyr mutation in six genetic haemochromatosis patients with distinct haplotypes indicates mutations within the HLA-H gene or at alternative genetic loci are the cause of genetic haemochromatosis in these patients. CONCLUSIONS: The HLA-H Cys282Tyr defect is likely to play a key role in the pathogenesis of haemochromatosis in most patients. Predominance of a single HLA-H gene mutation in haemochromatosis allows presymptomatic screening by genotypic analysis.

Factor VIII gene mutations found by a comparative study of SSCP, DGGE and CMC and their analysis on a molecular model of factor VIII protein.

Screening of the factor VIII (FVIII) gene which spans 186 kb and codes for 26 exons, was originally hampered by its size but is now feasible because rapid DNA scanning methodologies have been developed. The present study for the first time directly compares the three most widely applied screening methods, denaturing gradient gel electrophoresis (DGGE), single-stranded conformational polymorphism (SSCP) and chemical mismatch cleavage (CMC) for their sensitivity of mutation detection in a selected group of ten haemophilia A patients. Nine of these patients are known to be cross-reacting material positive and eight exhibited a mild to moderate phenotype. Of the ten patients screened, we identified mutations in nine by all three screening methods. Of the mutations characterised, two are previously unpublished. T to C (S373P) and G to A (D525N). In one mildly affected haemophiliac, we identified a second T to C sequence change in the 5' untranslated region at -601 bp, probably having no effect on FVIII gene expression. Modelling studies were performed on those mutations lying within the A domains of FVIII (D525N, R527W, I566T) to study the possible effect of these mutations on structure and/or function. When the three methods are performing optimally and have been standardised, our experience is that CMC and DGGE are equally efficient at sequence variation detection while SSCP is slightly less sensitive.

Prenatal diagnosis of triosephosphate isomerase deficiency.

First-trimester prenatal diagnosis was undertaken by chorionic villus DNA analysis in two unrelated families with the inherited glycolytic disorder triosephosphate isomerase (TPI) deficiency. The propositus in each family was shown to be homozygous for a missense mutation (GAG --> GAC) at codon 104 of the TPI gene. In the first case the fetus was heterozygous for the codon 104 mutation and therefore clinically unaffected. Prenatal diagnosis in the second case showed the fetus to be homozygous for the codon 104 mutation and thus affected by TPI deficiency. This represents the first molecular diagnosis during early pregnancy of a human glycolytic enzyme disorder.

Haemophilia A diagnosis by automated fluorescent DNA detection of ten factor VIII intron 13 dinucleotide repeat alleles.

Haemophilia A is a recessive X linked bleeding disorder caused by deficiency or functional abnormality of coagulation factor VIII. This disease usually has no visible phenotype in female carriers; hence, great efforts are made to offer all haemophilia A families accurate carrier diagnosis. Significant progress in this direction was made with the identification of the intron 13 variable number tandem repeat (VNTR), which is hitherto the most informative single marker within the factor VIII gene. The authors have established intron 13 VNTR detection in their laboratory by adapting its analysis to an automated sequencer using different primers of which one is fluorescent dye labelled. With this method, which is more rapid and convenient than that originally described, 67 haemophilia A families of German origin were screened and two new alleles (alleles 17 and 25) were identified. The informativeness of the VNTR in these families based on the patients maternal X chromosomes (134) is about 67%.

Analysis of the essential sequences of the factor VIII gene in twelve haemophilia A patients by single-stranded conformation polymorphism.

We report the analysis by single-stranded conformation polymorphism of the essential sequences of the factor VIII(FVIII) gene (total length about 14 kb) including the entire coding sequence, flanking intronic sequences and the putative regulatory sequences 5' to the gene, in twelve unselected haemophilia A patients of Portuguese origin. Direct sequencing of the fragments with an altered migration pattern led to the identification of the disease-producing mutations in five patients. Three of these mutations, namely a 1 bp insertion in a motif of eight consecutive A residues at codon 1439 (FVIIIPorto3); a C to T transition at codon 1966 (Arg-->Stop), found in an inhibitor-positive patient (FVIIIMontijo); and a G to A transition at codon 479 (Gly-->Arg; FVIIIPorto1), have been reported in other ethnic groups. The two novel mutations are the substitution of AG by GG at the 3' end of intron 4 (FVIIILisboa1) destroying the invariant splice acceptor sequence, and a G to A transition at codon 1948 resulting in an aspartic acid substitution for glycine (FVIIIPorto2).

Haemophilia A diagnosis by simultaneous analysis of two variable dinucleotide tandem repeats within the factor VIII gene.

Haemophilia A is a bleeding disorder caused by defects in the gene coding for the co-factor, factor VIII (FVIII). The few available intragenic restriction fragment length polymorphisms (RFLPs) currently used in carrier detection and prenatal diagnosis of haemophilia A are informative in only about 65% of cases. We earlier reported a multi-allelic dinucleotide tandem repeat, (CA)n, specific to intron 13, which remains the single most informative marker within the FVIII gene. We here report a second informative dinucleotide repeat of the form (GT)n (AG)n, located to intron 22 of the FVIII gene. The polymerase chain reaction (PCR) method was used to examine the variability of the repeat in 60 individuals (75 X-chromosomes) and revealed four alleles. The calculated heterozygosity rate is 45%, and family studies showed X-linked mendelian inheritance. The intron 22 dinucleotide repeat is tightly linked with established RFLPs and tracks with haemophilia A in family studies. We now show that by simultaneous amplification of the intron 13 and 22 repeats using PCR all alleles for both markers are detectable on a single polyacrylamide gel. The information thus obtained from a single multiplexed analysis is greater than from multiple RFLP analyses. Hence, rapid haplotype determination by simultaneous amplification and detection of two intragenic dinucleotide repeats should supersede less informative RFLP analysis.