Short Tandem Repeat (STR)
In 1994, ESR adopted short tandem repeat, or STR profiling. Although significant advances in DNA technology have been made since the introduction of this technique, most forensic DNA work throughout the world is still performed using variations of the original STR method.
STRs are short sequences of bases that are repeated end to end several times over, at many different sites (or loci) on the DNA helix. Different people tend to have different numbers of the repeat unit at each locus, and it is this difference that allows individuals to be told apart from each other by STR profiling.
The number of STR repeat units that a person has at a certain locus are counted, and the number of repeats is referred to as the allele for that locus. If two DNA samples have different alleles at a locus, then a forensic scientist can be certain that the samples came from two different people.
While there may only be ten to twenty possible alleles at each locus, modern DNA tests use multiple loci that, when used in combination, produce a test that can provide very high levels of discrimination. ESR currently uses the Identifiler® testing system containing fifteen STR loci, in addition to a sex test based on the Amelogenin gene.
The chance of obtaining a random match between two individuals using this system has been estimated at less than one in 1012 (an English billion).
Other DNA tests using STR-based technology include
- Low Copy Number (LCN) - enables scientists to produce DNA profiles from samples that contain only a few cells, such as the cellular material deposited in a fingerprint.
- Y-STR - Short Tandem Repeats (STRs) found on the male-specific Y Chromosome.
- Mini STR - increases the likelihood of obtaining STR results from samples with compromised DNA, inhibitors and degradation.