Genetic instability is one of the most characteristic features of cancer cells. It is expressed in the cell accumulation of chromosomal aberrations and mutation, as well as microsatellite instability and loss of heterozygosity. Microsatellite instability (MSI) is a mutator phenotype characteristic. It occurs in hereditary and sporadic cancers, although with different frequencies. MSI is to change the length (size) alleles by increasing or reducing the number of nucleotide repeats.