Following a rape incident in an apartment in Japan, we were requested to perform a DNA analysis on a body fluid stain left on a bath towel to determine whether it could be attributed to the suspect. The acid phosphatase and prostatic-specific antigen tests confirmed it to be a seminal stain. Based on the DNA analysis by autosomal and Y-chromosome short tandem repeat (STR) systems, no inconsistency was found with the profile of the suspect with African ancestry. In this case, allele 21 of DYS390 at the Y-STR locus was examined, as it is reported to have a distinctly lower frequency in the Japanese population. Furthermore, the haplotype combinations of Y-STR at the DYS389I, DYS389II and DYS390 loci are powerful for personal identification, as these have not yet been found in the Japanese population.