Cancer is a genetic disease. Cancer cells contain various mutations, which includes SNPs to chromosomal aberrations. Together, these changes are referred to as genome instability. Genetic instability is one of the common characteristics of colorectal cancer. In colorectal cancer three major types of genetic instability have been reported. They are chromosomal translocations, microsatellite instability (MSI), and chromosome instability (CIN). Microsatellite instability occurs due to variations in DNA mismatch repair genes, while chromosomal instability is distinguished by major chromosomal alterations occurring at cell division and usually involves β‐catenin and Adenomatous polyposis coli protein (APC) mutations. This chapter summarizes the major molecular mechanisms leading to genomic and microsatellite instability and tumorigenesis
