Cell proliferation is a highly regulated process in which two classes of genes play crucial roles, proto-oncogenes and tumor suppressor genes. Proto-oncogenes induce cell proliferation, and tumor suppressor genes control the levels of proto-oncogenic expression, or the period of activity of proteins transcribed by proto-oncogenes, thus keeping cell proliferation within normal rates. Since uncontrolled cell proliferation leads to tumor formation, genes that prevent abnormal cell proliferation are termed tumor suppressor genes.

When a tumor suppressor gene is mutated or inactivated, cells become either displastic or malignant. When only one copy or allele of a tumor suppressor gene is mutated, dysplasia, or benign overgrowth, usually occurs, due to a lower level of expression of anti proliferative proteins. When a second mutation occurs in the other allele of the same gene, the onset of cancer triggers. Some examples of tumor suppressor genes that are found mutated...