Genes which, when homozygous mutant, result in cancer. Such genes code for protein products that inhibit cell proliferation.

Genes that when inactivated enhance carcinogenesis.

The role of tumor suppressor genes is to maintain the integrity of the DNA in cells. The activation of a tumor suppressor gene at certain "check points" puts on the "brakes" and allows the cell to check for any damage in its DNA. Usually repairs are made before the cell is allowed to go on and divide. Without these brakes, cells with damaged DNA would copy the damage and pass damage on to daughter cells. The damage becomes a permanent mutation in future generations of new cells.