Unusual variant alleles in commonly used short tandem repeat loci.

Unusually large variant alleles were observed in the short tandem repeat (STR) systems D3S1358 and D21S11, both of which are included in the international standard set of loci (ISSOL) and routinely typed in National DNA intelligence databases worldwide. The observed alleles fell within the size range of the adjacent STR marker, which could easily cause problems with respect to correct allele assignments for both loci concerned. We compared the amplification and potential interpretation with three different commercially available kits, which are frequently used in forensic work. PCR products were cloned and sequenced in order to determine the structure of these unusual allele variants and confirm their size and designation (D3S1358 allele 26, D21S11 allele 46). In the locus D21S11 we observed an as yet undescribed partial duplication of the constant region.