M. B. KASTAN Cancer is a disease resulting from alterations of cellular genes which cause phe- notypic changes in somatic cells. Usually, when we think about genetic diseases, we think about inheriting one or two abnormal genes from our parents and these gene abnormalities confer the disease phenotype. In contrast, in the majority of cancers, no such inherited gene abnormalities can be identified (which does not mean that they do not exist) and there is no obvious family history suggesting an inherited disease. The vast majority of genes which are altered in the cancer cells are not...