The genome contains repetitive sequences spanning 500 to 20,000 base pairs (a repeat unit is 5 – 30 base pairs); these sequences are called minisatellite or variable number of tandem repeats (VNTR). They are mainly located near chromosome ends. In some minisatellite sequences, frequent changes of the repeat number (up to 10%) are observed in germ cells. Such alterations may occur a hundred to a thousand times more frequently than mutations in a DNA region coding for a protein.