PCR-based DNA typing of saliva on stamps and envelopes

In forensic cases involving mail bombs, extortion, kidnapping or threatening letters, biological evidence such as the saliva used to attach the stamp and seal the envelope could be used for genetic analysis. We have developed a highly sensitive semi-nested PCR method for the HLA-DRB1 locus; suitable for the analyses of very limited amounts of DNA. When applied to a set of stamps and envelopes with saliva from control individuals, typing results were consistent with those obtained using hairs drawn from the same individuals. No interference was found due to DNA from the fingerprints of people handling the letters. The system was applied to three forensic cases with threatening letters. The first case resulted in an exclusion of the suspect. In the second case, the suspect could not be excluded (probability of identical genotype by chance > 0.01). These results demonstrate that biological evidence in cases with threatening letters is amenable to genetic typing.