The most common form of DNA analysis is called polymerase chain reaction (PCR). PCR has allowed investigators to successfully analyze evidence samples of limited quality and quantity. The PCR process makes millions of copies of very small amounts of DNA. This enables the laboratory to generate a DNA profile, which can be compared with the DNA profile from a suspect. A statistic is then generated to reflect how often one would expect to find this particular DNA profile in the general population.

Three types of results can occur in DNA testing: inclusion, exclusion, and inconclusive. It is important that victim service providers understand the meaning of these terms and be able to explain their implications.

Inclusion. When the DNA profile of a victim or suspect is consistent with the DNA profile from the crime scene evidence, the individual is "included" as the possible source of that evidence. However, the strength of inclusion depends upon the number of loci (locations on the DNA strand) examined and how common or rare the resulting DNA profile is in the general population.

Exclusion. When the DNA profile from a victim or suspect is inconsistent with the DNA profile generated from the crime scene evidence, the individual is "excluded" as the donor of the evidence. However, exclusion does not imply innocence. In a rape case, for example, a perpetrator wearing a condom could be excluded as a suspect because no semen was found at the crime scene, but evidence found elsewhere at the crime scene may include that same person as a suspect.

Inconclusive. Inconclusive results indicate that DNA testing could neither include nor exclude an individual as the source of biological evidence. Inconclusive results can occur for many reasons: for example, the quality or quantity of DNA may be insufficient to produce interpretable results, or the evidentiary sample may contain a mixture of DNA from several individuals (e.g., a sample taken from a victim of a gang rape).

As with all DNA testing results, additional testing may be needed and findings should be interpreted in the context of other evidence in the case.