Digital DNA typing at a second hypervariable locus by minisatellite variant repeat mapping.

Minisatellite variant repeat unit mapping by PCR (MVR-PCR) assays the interspersion pattern of variant repeat units along minisatellite alleles. Mapping such internal variation in the highly polymorphic minisatellite MS31A (locus D7S21), reveals extreme levels of allelic variability, far in excess of that detectable by allele length analysis. Flanking base substitutional polymorphisms have enabled the 5' structure of large numbers of MS31A alleles to be derived from genomic DNA by allele-specific MVR-PCR. More than 100 alleles have now been mapped and all are different. Several alleles show related internal structures and some of these provide evidence of polarity in allelic variation reminiscent of that seen at two other hypervariable minisatellites, D1S8 (MS32) and D16S309 (MS205). We also describe the diploid digital coding of MS31A, including the simultaneous coding of MS31A and a second locus, MS32, by duplex MVR-PCR, which greatly enhances the potential forensic applications of this technique.