During the course of paternity test, three samples in two cases were apparently homozygous at the CSF1PO locus using AmpFlSTRs Identifiler PCR Amplification kits, but using the PowerPlexs 16 kit, the three individuals were found to be heterozygous. This puzzling problem was solved by using multiple analytical approaches, including the use of different primer pairs and the characterization of the mutation causing the ''null allele.'' Dropout was caused by a single mutation event in the presumptive binding site of the forward primer. While the frequency of these silent alleles remains low (0.5% 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.