Cancer is a genetic disease but is only rarely inherited. Most cancers are sporadic and arise in a particular tissue such as the colon, breast, lung or skin, following exposure of the normal tissue to carcinogens that cause somatic mutations in one or more oncogenes or tumor suppressor genes. Familial cancer syndromes, which in total account for less than one percent of all cancer, occur in individuals who have inherited a germline mutation in a tumor suppressor gene. Most familial cancers have autosomal dominant inheritance and are characterized by the development of cancer at a young age.

Inheriting a mutant tumor suppressor gene effectively knocks out one allele of the gene (i.e., the "first hit") in every cell in the body. This leaves the individual vulnerable to a "second hit" on the remaining normal gene allele. Some tissues or body organs such as the eye...