The use of DNA (deoxyribonucleic acid) as a method of identification is relatively new, but it has proven an effective means of identifying criminals and perhaps more important, eliminating people as crime suspects. A fingerprint is the only unique identification source (identical twins have the same DNA). But if a criminal leaves no prints behind, law enforcement officials must rely on minute DNA samples from blood, saliva and other bodily fluids, hair, or skin. DNA testing is also used in paternity disputes to determine the identity of the actual father in custody, inheritance, or child support suits.
DNA testing can be done by standard techniques such as restrictive fragment length polymorphisms (RFLP), polymerase chain reaction (PCR), short tandem repeat (STR), and mitochondrial analysis. In RFLP testing, a DNA sample is mixed with a chemical substance that helps examiners isolate and identify specific key fragments of the sample that can be used in comparison analysis. A drawback of RFLP is that it requires a fairly large DNA sample. With PCR, a series of chemical reactions helps generate copies of a minute DNA sample, thus amplifying a small or degraded piece of information. In STR, various DNA regions in a sample are compared with other samples for similarities. The FBI uses STR using special software that can identity thirteen of these regions in a DNA sample. Mitochondrial DNA analysis is often used for extracting samples from bones and teeth, for which the other methods are not effective.
The FBI keeps a computerized databank of DNA samples called CODIS (Combined DNA Index System), which contained about 1.7 million DNA profiles as of 2003. The profiles stored in CODIS can be used to convict criminals, and also to exonerate innocent people. There are numerous examples of criminals whose DNA matched a profile from an earlier crime and who were then charged with the crime; likewise, there are examples of individuals whose innocence was confirmed when DNA found at a crime scene turned out to belong to another person identified through the profiles.