Short tandem repeats are repeated DNA sequences that can be used by forensic investigators to identify people and remains. By looking at the number of repeat segments across areas of the DNA strand, investigators can determine whether a DNA profile is unique and distinguish among individuals, helping solve missing persons cases and identifying people convicted of or arrested for crimes. Short tandem repeats can also be used to resolve more than one case by connecting separate cases or connecting a case to an individual.
Source: National Institute of Justice
Welcome to NIJ’s Term of the Month. Each month we are featuring a term from our scientific research portfolios informing significant American justice system issues and solutions.
July 2024 — Short Tandem Repeats
DNA is made up of four bases, known collectively as C, T, A, and G. These bases are arranged in long strings which pair together to create double strand DNA. Short tandem repeats (STRs) are DNA sequences that can be used to identify people. Specifically, STRs are multiple copies of an identical (or similar) DNA sequence arranged in direct succession where the repeat sequence unit is two base pairs to six base pairs in length.[1] By looking at the number of repeat segments across multiple areas of the DNA, investigators can determine whether a DNA profile is unique and, therefore, able to identify an individual.
Current estimates indicate that 1%-3% of DNA is the same in all people. This percentage of DNA acts the same for everybody: it encodes precise, universal proteins that define us as healthy humans. However, in the other 97%-99%, the DNA sequence does not create important proteins or do anything else of genetic importance that we recognize. STRs occur in these areas; because they have no known functional effect, they vary greatly from person to person.
By comparing STRs, forensic investigators can use DNA to distinguish among individuals. In one person, for example, there could be a five-repeat sequence of CTAG. At the same site on another person’s DNA, the CTAG sequence could repeat six or seven times. The variation among individuals is enormous, making it useful for identification.
When collecting DNA evidence in forensic investigations of a crime, the FBI has designated 20 of the most common STR loci (specific DNA regions) to serve as the standard for its Combined DNA Index System (CODIS). CODIS is a database containing DNA profiles from persons convicted of or arrested for crimes, unsolved crime scene evidence, and missing persons. The combination of STRs across the 20 loci vary tremendously. Each unique combination creates an individual DNA profile. This means CODIS examines millions of possible STR combinations when investigators are looking for a DNA match. Because of these myriad combinations, the odds of finding a match (match rarity) in the 20 loci is at least 1 in 14 million and is often much higher. By establishing the 20 STR loci standard, the FBI system ensures that all forensic laboratories have a uniform database for using and sharing information.
This uniformity makes DNA matching more efficient, and it also means that STRs can potentially resolve more than one case. When DNA evidence is collected at a crime scene or from a victim of a crime, a DNA profile is developed and entered into CODIS. A CODIS search is then performed and can result in two types of associations. The first is with a DNA profile from another crime and the second is with a DNA profile from a convicted or arrested individual. This provides two types of investigative leads, one connecting the crime scene sample to another crime scene and the other to a convicted person. The result provides information that has the potential to either connect cases to each other or a case to an individual.
