We experienced a difficult case of TH01 typing. Instability of TH01 allele 9.3 was observed using GenePrint STR System TH01. Allele dropout was observed when an AmpFlSTR Profiler Kit was subsequently used for confirmation of the TH01 type. Use of the PowerPlex 16 System made it possible to detect allele 9.3. As a result of sequencing, a single point mutation (G-to-A transition) located 37 bases upstream of the first TCAT motif of the repeat region was identified as the cause of the allele dropout during use of the AmpFlSTR Profiler Kit. This mutation was located at the 3' end of the forward primers of the AmpFlSTR Profiler Kit and GenePrint STR System TH01.