Microsatellites are one of the most convenient and widely used markers for DNA identification. This is a special class of DNA markers, which are DNA fragments with a large number – up to a hundred and more – of identical tandem repeating “motifs”. The motif is a short sequence of several (2 to 8) base pairs, commonly referred to as a “repeat”.

Depending on the repeat length, microsatellites are classified into loci with di-, tri-, tetra-, penta-, and hexanucleotide repeats. Thus, a microsatellite, or microsatellite locus (STR-locus, Short Tandem Reapets), is called a DNA segment located on a particular chromosome and containing short tandem repeats.

Alleles of the microsatellite locus differ from each other in the number of repeats and, as a consequence, in length. Microsatellite loci are highly polymorphic – that is, there are many alleles for each of them. For example, the locus FCA149, located in the B1 chromosome of the domestic cat, contains TG dinucleotide repeats. In the populations of F. catus, 6 alleles of this locus were found (with the number of repeats from 13 to 18). This allele of the FCA149 locus has 17 TG repeats. Therefore, the formula for this locus is written as FCA149.