STR (Short Tandem Repeat) markers are polymorphic sequences found evenly spaced in mammalian genomes.

The core repeats can be di- (two bases, e.g., CA) tri-(three bases, e.g., CAG) or tetra-nucleotides (four bases, e.g., ATGC). These core sequences are repeated a number of times after each other (e.g., CACACACACACA or CA in parethesis times 6).

Different alleles have different numbers of repeats in the individual STR loci. In the population, multiple alleles can be present (e.g., five different alleles), but in an individual(diploid), there is a maximum of two different alleles (one on each homologous chromosome).

The STR-loci are flanked by unique sequences, and therefore the individual STR-loci can be amplified by PCR and the sizes of the alleles estimated by gel electrophoresis. In this manner genotyping of the individual markers can be performed.