Short Tandem Repeats (STRs) are regions of the genome composed of approximately 1-5 bases and repeated up to 17 times. STR markers will either be simple (identical length repeats), compound (two or more adjacent repeats) or complex (several different length repeats). They are found on 22 autosomal chromosomes as well as both X and Y sex chromosomes, though those on the Y chromosome differ less due to lack of recombination. Only a select number of STR markers are used in forensic DNA profiling (10 in the UK and 13 in the US).They express a high degree of polymorphism, making them of particular use to the forensic scientist. The variability in STRs is caused by the inaccuracy of DNA polymerase in copying the region. As STR regions are non-coding, there is no selective pressure against the high mutation rate, resulting in high variation between different people.
Though there have been thousands of short tandem repeats found in the human genome, only a small number are utilised in forensic DNA analysis. STRs used in forensic science tend to be tetra- and penta-nucleotide repeats, as they are both robust, suffer less environmental degradation, and provide a high degree of error free data. STR loci are ideal for use in forensic science for a number of reasons. They represent discrete alleles that are distinguishable from one another, they show a great power of discrimination, only a small amount of sample is required due to the short length of STRs, PCR amplification is robust and multiple PCR can be used, and there are low levels of artefact formation during amplification.