During duplicate analysis of buccal swabs from 1,377 individuals with 2 commercial short tandem repeat (STR) kits, we observed 8 discordant phenotypes with SGM Plus (SGM, second generation multiplex) for the STRs THO1 (2), vWA (4) and D18S51 (2), and 1 discrepancy with PowerPlex 16 for D18S51. One individual even showed two discrepancies (vWA and THO1) for SGM Plus. In each case, the difference observed was due to the non-amplification or allele dropout of the second allele in a heterozygous genotype. Sequence analysis revealed each time the presence of a mutation that probably coincided with the primer-binding site. Primer-binding site mutations for vWA and D18S51 have been reported previously, while the mutation for THO1 (C-to-T substitution at position 1286 of GenBank sequence D00269) is reported here for the first time. While the frequency of these silent alleles remains low (0.58% in our study), it is suggested that appropriate measures should be taken for database comparisons and that allelic dropout should be further investigated by sequence analysis and be reported to the forensic community.