Short sequences of nucleotides in DNA, 2–6 bp long, that are repeated in tandem at particular loci throughout the genome and constitute microsatellite DNA. The number of repeats at any one site varies typically between five and thirty. STRs are widely used as the basis for DNA profiling because they are highly variable and their small size makes them readily amenable to detection and amplification by the polymerase chain reaction. See also repetitive DNA.