A novel type of three band pattern at STR loci

SE33 or ACTBP2 is the most polymorphic locus in many national DNA databases and in the commercial STR kits used to type both crime scene samples and reference samples to populate these databases. We describe the molecular reason for a three band pattern of SE33 seen in several samples. A SNP in the flanking SE33 region causes the binding of the unlabelled D3S1358 primer. As a result, a "chimeric" PCR product of the labelled SE33 primer and the D3S1358 primer is generated that is smaller than the regular SE33 amplicon. We call this "Type 3 three band pattern" as the genetic base differs from the Type 1 three band pattern caused by somatic mosaicism and the Type 2 that results from copy number variation.