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Unformatted text preview: Peter Gill Forensic Science Service, Birmingham, UK ABSTRACT The analysis of short tandem repeat (STR) DNA sequences is of fundamental im- portance to forensic science because they have become the recognized standard in constructing national public databases. Consequently, considerable effort has been expended in developing multiplexed (one tube) reactions that analyze several loci in combination. The implementation of STRs in casework cannot take place without a full understanding of the systems used. The pur- pose of validation is to characterize multi- plexes when one is challenged with forensic samples. For example, mixtures are often encountered that may be particularly diffi- cult to interpret against a background of al- lelic artifacts. By increasing the number of PCR ampli- fication cycles, it is possible to dramatically boost the sensitivity of the system so that just a handful of cells may be successfully analyzed. However, interpretation is much more complex because the origin of DNA profiles may be less certain and complicat- ed by issues such as contamination, the po- tential for innocent transfer, and a predomi- nance of mixtures. This review provides a brief historical background of the development of STRs in forensic casework that culminated in the creation of national DNA databases. The development of guidelines to interpret com- plex DNA profiles, such as mixtures, is out- lined. Finally, the recent innovation of low copy number DNA profiling is explained along with the special considerations need- ed to report in court. DEVELOPMENT OF MULTIPLEXED SYSTEMS Early multiplexes consisted of few loci that were based on simple short tan- dem repeats (STRs). The four-locus “quadruplex” was probably the first to be widely used (44); because it consist- ed of few STRs, the match probability was consequently high—1 in 10 000. In 1996, a six-locus STR system (57,58) combined with the amelogenin sex test (61) was introduced—known as the “second generation multiplex” (SGM). Because this system utilized complex STRs D21S11 and HUMFIBRA/FGA (47), which have greater variability than simple STRs, the match probability was consequently decreased to 1:50 million. In the UK, the introduction of SGM co- incided with the implementation of the UK National DNA Database (75). More than a million samples are now stored on the database. As databases become much larger (numbering several mil- lions), it is necessary to ensure that the match probability of the system is suffi- cient to minimize the chance of two un- related individuals matching accidental- ly. Consequently, a new system known as the Amp Fl STR SGM Plus (Ap- plied Biosystems, Foster City, CA, USA) (18) was introduced in 1999 and comprised 10 STR loci and amelogenin, replacing the previous SGM system....
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