Variations in primer sequences are the origin of allele drop-out at loci D13S317 and CD4

STRs have become almost the exclusive tool of genetic scientists in forensi c typing work. Consequently, large numbers of samples are genotyped and the detection of rare abnormalities is to be expected. We found rare losses of alleles, also known as drop-out, at the two STR loci D13S317 and CD4. Drop -out at D135317 was accidentally found in typing of suspects in a murder ca se and three other examples of drop-out were found at locus CD4 during pate rnity testing. The lost alleles reappeared when alternative PCR primer pair s were used. Sequences of lost alleles were characterised at the molecular level after cloning. Variations were found in the primer sequences and thes e are believed to prevent amplification or to reduce amplification yield an d to be the origin of the allele drop-out.