This case shows the importance of the use of a large number of STRs in resolving a priori a paternity dispute when a close relative of the alleged father could be the biological father of the child. Also, the publication of primer sequences for AmpFISTR kits would assist in the accurate identification of mutations that may be the cause of erroneous homozygosity (null alleles) and contradictory results from tests that use different primers. In the current case, the AmpFISTR Profiler Plus and AmpFISTR Identifiler PCR Amplification kits determined that the alleged father and the two children were apparently homozygous at the FGA locus; however, the PowerPlex 16 kit showed that the alleged father and 2 children were heterozygous. In addition, 3 inconsistencies were found between the daughter and the alleged father among 18 STR markers. The occurrence of the rare null allele at the FGA locus and case history suggested that the true father was the brother of the alleged father. A single-step repeat maternal mutation was also detected at D16S539. In order to investigate the presumed mutation in the primer-binding region, PCR amplification was performed with a new primer pair external to the amplicons generated by the AmpFISTR kits. The amplicon generated by this new primer pair had a length of 402bp for allele 24 compared with a length of 243bp with the Profiler Plus and the Identifiler kits. 1 table, 3 figures, and 19 references