In practice many LCN profiles are mixtures and difficult to interpret reliably due to the issues of allele dropout and drop-in described above. The success rate for obtaining a clean profile can be poor due to the limited amount of sample available. Thus a lot of work can go into generating DNA profiles that may not be probative or useful in a particular forensic case. Although a DNA profile may be obtained, it is usually not possible to identify the type of cells from which the DNA originated or when the cells were deposited (Gill 2001). Furthermore, with such little starting material, it may not be possible to preserve LCN evidence to enable confirmatory testing by a second laboratory should that be required. Nevertheless, even with these caveats LCN results have enabled recovery of DNA profiles from burglaries and other situations where only a few cells from the perpetrator were present and have thereby extended the power of DNA testing.
In some cases, it may be possible to obtain reliable results from low levels of DNA without having to boost the PCR cycle number and push the sensitivity of the amplification. Budowle et al. (2001) list several alternatives to LCN to enable boosting a signal for a STR profile without increasing the PCR cycle number and the concomitant increased risk of contamination. These alternatives include: (1) reducing the PCR volume to get a more concentrated PCR product, (2) filtration of the PCR product to remove ions that compete with the STR amplicons when being injected into the capillary (see Chapter 14), (3) use of a formamide with lower conductivity, (4) adding more amplified product to the analysis tube, and (5) increasing the injection time on the capillary electrophoresis instrument.
Some important LCN reporting guidelines include: (1) multiple tube PCR amplifications with demonstrated duplication of every allele before reporting results, (2) if negative controls associated with a particular batch of samples show duplicated alleles that correspond to any of the samples, then the samples should not be reported and where possible samples should be retested, and (3) if there is one allele in a sample that does not match the suspect's STR profile, then further testing may be pursued (Gill et al. 2000). Generating STR data with an increased number of PCR cycles and invoking a LCN philosophy can provide a useful lead in many instances for an investigation but it is unlikely to provide definitive probative evidence of a crime in every instance.
In this chapter we have focused on forensic issues with STR typing. The final section will cover other uses for STR typing that involve the capability of these markers for mixture detection.
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