A high-density SNP map for the FRAX region of the X chromosome. Single-nucleotide polymorphisms.

Single-nucleotide polymorphisms (SNPs) are the most common type of genetic variation within the human genome, occurring approximately once every kilobase. However, for association studies, SNPs are not as informative as microsatellite markers and a large number of SNPs and substantial population sizes are required for linkage and mapping studies. A SNP map was generated for the FRAX region of the X chromosome, approximately 0.8 Mb proximal and 1.8 Mb distal to the FRAXA repeat, at a density of at least 1 SNP every 100 kb. SNPs were identified in a population of 28 women with a FRAXA expan-sion (including three women with a FRAXE expansion) on a background of different DXS548, CA1 and CA2 haplotypes, and a normal X chromosome with a different microsatellite haplotype. Fifty-four polymorphisms were identified in a total of 52 257 bp distributed over 2.6 Mb. This represented about 1 SNP every 1024 bp, which was consistent with a nondesert region (1 : 1000 bp). Because the SNPs identified in this study have haplotype and frequency data from an affected population, they should provide a useful resource for researchers to investigate the genetic mechanisms behind instability and expansion of both FRAXA and FRAXE triplet repeats.