Identification of a rare mutation in a TH01 primer binding site

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.