STR analysis of artificially degraded DNA--results of a collaborative European exercise.

Degradation of human DNA extracted fromforensic stains is, in most cases, the result of a natural process due to the exposure of the stain samples to the environment. Experienceswith degraded DNAfrom casework samples show that every samplemay exhibit different properties in this respect, and that it is difficult to systematically assess the performance of routinely used typing systems for the analysis of degraded DNA samples. Using a batch of artificially degraded DNAwith an average fragment size of approx. 200 bp a collaborative exercise was carried out among 38 forensic laboratories from 17 European countries. The results were assessed according to correct allele detection, peak height and balance as well as the occurrence of artefacts. A number of common problems were identified based on these results such as strong peak imbalance in heterozygous genotypes for the larger short tandemrepeat (STR) fragments after increased PCRcycle numbers, artefact signals and allelic drop-out. Based on the observations, strategies are discussed to overcome these problems.