OBJECTIVETo explore the cause for allelic drop-out at short tandem repeat (STR) loci upon paternity testing with a PowerPlex® 16 kit.

METHODSA total of 10 642 DNA confirmed paternity testing cases (18 314 parent/child allelic transfers) were analyzed with the PowerPlex® 16 kit. Samples suspected for having allelic drop-out were verified with an Identifiler™ kit and/or locus-specific singleplex amplification systems. PCR products of null alleles were separated and directly sequenced.

RESULTSEight cases of allelic drop-out were found. The overall rate of null allele in the PowerPlex® 16 system was 0.437 × 10(-3). DNA sequencing has confirmed single base variations within the binding region of published primers, in which 4 cases involved the D18S51 locus (2 cases with G>A transitions at 79 bp upstream of the repeats, 1 case with G>T transversion at 162 bp downstream of the repeats and 1 case with G>C transversion at 74 bp upstream of the repeats), 2 cases involved the D21S11 locus (1 case with C>A transversion at 17 bp upstream of the repeats and 1 case with A>G transition at 12 bp upstream of the repeats). One case involved the FGA locus (1 case with G>A transition at 142 bp downstream of the repeats) and 1 case involved TPOX locus (1 case with G>A transition at 198 bp downstream of the repeats).

CONCLUSIONBase variation in the primer binding region may cause failed PCR and result in null allele reports. Alternative primer sets should be used to verify the suspected allelic drop-out. Attention should be paid to this during paternity testing and data exchange for personal identification.