Short tandem repeat (STR) technology is a forensic analysis that evaluates specific regions (loci) found on nuclear DNA. The variable (polymorphic) nature of the STR regions that are analyzed for forensic testing intensifies the discrimination between one DNA profile and another. For example, the likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater.
The Federal Bureau of Investigation (FBI) has chosen 13 specific STR loci to serve as the standard for the Combined DNA Index System (CODIS). The purpose of establishing a core set of STR loci is to make sure that all forensic laboratories can establish uniform DNA databases and, more importantly, share valuable forensic information.
If the forensic or convicted offender CODIS index is to be used in the investigative stages of unsolved cases, DNA profiles must be generated by using STR technology and the specific 13 core STR loci selected by the FBI.
(Information provided by the Department of Justice)