3297-Why does the PCR for HLA-DRB4 result in 2 amplicons of different size?

In contrast to HLA-DRB3 and HLA-DRB5, the primer set for HLA-DRB4 used in the SBTexcellerator HLA-DRB3/4/5 Kit amplifies exons 2 and 3 at this locus separately in a single reaction, resulting in two PCR amplicons.