Three-step maternal mutation in locus TPOX detected by STR analysis.

A three-step maternal mutation in TPOX locus was observed by autosomal STR genotyping in routine paternity testing involving a mother-daughter pair. The results found that the duplex individuals matched at 29 loci detected except for an inconsistency at locus TPOX. The mother's genotyping in the locus is homozygotic with allele 11, while the child's genotyping is homozygotic with allele 8. The massively parallel sequencing analysis showed that core motif of TPOX locus is AATG[11] in mother and AATG[8] in sister. Clearly, this was a three-step maternal mutation at TPOX locus. Although STR mutations involving one, two, or even four steps have been reported with the overall average mutation rate ranging from 0.001 to 0.004, the forensic experts still pay attention to the multistep mutations in routine paternity testing.