cepted as valid for forensic use, it must be rigorously characterized in both research and forensic settings to determine the circumstances under which it will and will not yield reliable results. It is meaningless to speak of the reliability of DNA typing in general—i.e., without specifying a particular method. Some states have adopted vaguely worded statutes regarding admissibility of DNA typing results without specifying the methods intended to be covered. Such laws obviously were intended to cover only conventional RFLP analysis of single-locus probes on Southern blots—the only method in common use at the time of passage of the legislation. We trust that courts will recognize the limitations inherent in such statutes.

Forensic DNA analysis should be governed by the highest standards of scientific rigor in analysis and interpretation. Such high standards are appropriate for two reasons: the probative power of DNA typing can be so great that it can outweigh all other evidence in a trial; and the procedures for DNA typing are complex, and judges and juries cannot properly weigh and evaluate conclusions based on differing standards of rigor.

The committee cannot provide comprehensive technical descriptions for DNA typing in this report: too many methods exist or are planned, and too many issues must be addressed in detail for each method. Instead, our main goal is to provide a general framework for the evaluation of any DNA typing method.

ESSENTIALS OF A FORENSIC DNA TYPING PROCEDURE

Scientific Foundations

The forensic use of DNA typing is an outgrowth of its medical diagnostic use—analysis of disease-causing genes based on comparison of a patient's DNA with that of family members to study inheritance patterns of genes or with reference standards to detect mutations. To understand the challenges involved in such technology transfer, it is instructive to compare forensic DNA typing with DNA diagnostics.

DNA diagnostics usually involves clean tissue samples from known sources. It can usually be repeated to resolve ambiguities. It involves comparison of discrete alternatives (e.g., which of two alleles did a child inherit from a parent?) and thus includes built-in consistency checks against artifacts. It requires no knowledge of the distribution of patterns in the general population.

Forensic DNA typing often involves samples that are degraded, contaminated, or from multiple unknown sources. It sometimes cannot be repeated, because there is too little sample. It often involves matching of samples from a wide range of alternatives present in the population and thus lacks built-in consistency checks. Except in cases where the DNA evidence



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