laboratory's reproducibility. It would likely be a powerful tool for quality control in a laboratory. For convenience, blind control samples could be distributed by a professional association or a private-sector firm. The committee recommends that testing laboratories adopt such a system for continuing measurement of reproducibility and that they regularly examine and report the results. Recommendations for mandating such testing systems are discussed in Chapter 4.

Retention of Sample

Scientifically, the best way to resolve ambiguity is often to repeat the experiment. The U.S. justice system guarantees opposing sides the right to have repeat experiments performed by experts of their choice, whenever that is possible. Accordingly, testing laboratories should measure DNA samples before analysis (with accurate devices, such as fluorometers, as well as with ethidium-stained "yield" gels) and should use only the quantity of DNA required for reliable Southern blot analysis. When they can, they should retain enough of a sample to repeat the entire analysis.

TECHNICAL ISSUES IN PCR-BASED METHODS

PCR is a relatively new technique in molecular biology, having come into common use in research laboratories only in the last 4 years. Although the basic exponential amplification procedure is well understood, many technical details are not, including why some primer pairs amplify much better than others, why some loci cause systematically unfaithful amplification, and why some assays are much more sensitive to variations in conditions. Nonetheless, it is an extremely powerful technique that holds great promise for forensic applications because of its great sensitivity and the potential of its use on degraded DNA.

We discuss here two broad categories of technical issues concerning PCR methods: issues related to the amplification step and issues related to the detection of amplified product.

Technical Issues Related to Amplification

Amplification Conditions

The quality and specificity of amplification with PCR depends on the amplification conditions: the amplification cycling program (temperatures, mixture (e.g., primer, nucleotide, polymerase, and magnesium concentrations), times, and number of cycles), the composition of the amplification and the amount and nature of the target DNA in the sample (single-stranded or



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