THE VALIDATION OF A 7-LOCUS MULTIPLEX STR TEST FOR USE IN FORENSIC CASEWORK .2. ARTIFACTS, CASEWORK STUDIES AND SUCCESS RATES

PCR-based DNA typing of biological evidence is now widely used in fore nsic analyses due to the obvious advantages of enhanced sensitivity, t he ability to distinguish discrete alleles and efficacy with degraded samples. A multiplex short tandem repeat (STR) system has been previou sly developed which successfully co-amplifies six STR loci HUMTH01, D2 1S11, D18S51, D8S1179, HUMVWF31/A and HUMFIBRA (FGA) in conjunction wi th the X-Y homologous gene Amelogenin. This is known as the second gen eration multiplex system (SGM). Detection of the PCR products is under taken on ABD 373A or 377 automated sequencers using denaturing polyacr ylamide gels coupled with fluorescent-based technology. We have evalua ted this system for routine forensic use and demonstrated that the tec hnique is robust and reproducible under conditions consistent with tho se encountered in a forensic environment. A total of 132 stains from s imulated and actual casework were analysed, together with relevant con trol areas and reference samples. The success rate was high with 76% o f stains giving full profiles; we were also able to successfully detec t and interpret mixtures. No mistyping was observed. A detailed examin ation of each of these profiles has assisted in the development of gui delines for casework interpretation. Although artefacts, stutter peaks and undenatured DNA were occasionally observed, these did not interfe re with the accuracy of interpretation. In addition 38 samples, previo usly examined using the quadruplex system, were analysed with the SGM to enable a direct comparison to be made between the systems. The perf ormance of the system with poor quality samples demonstrated its use a s a rapid and powerful technique for individual identification.