Analysis of the polymorphic structure of the D7S808-short tandem repeat (STR) locus

We analyzed the polymorphic structure of the short tandem repeat (STR) (AARG) locus D7S808 by DNA sequencing and examined the D7S808 allele distribution in a Japanese population. The sequence analysis confirmed that this locus consists of repeats of the tetranucleotides cttt and cctt, but that the number of repeats of the cctt motif does not vary with the allele, and that this STR polymorphism is due to variation in the number of cttt repeats alone. Although the results in this study suggest that the numbers of repeats range from 7 (allele 7) to 22 (allele 22), alleles 9, 10, 19, and 21 were not observed in the Japanese samples examined. Analysis of DNA samples from 355 unrelated individuals revealed the occurrence of 286 heterozygotes (observed heterozygosity 80.6%). Alleles 15, 14, 16, and 17 had high frequencies of 0.261, 0.192, 0.166, and 0.120, respectively and, together with allele 7 with a slightly high frequency of 0.059, showed a bimodal distribution. In addition, we prepared primers yielding shorter amplification products (232-292 bp) than those (435-480 bp) obtained with the originally reported primers. The newly designed primers can be used for polymerase chain reaction, making this locus extremely useful in forensic science practice.