A predisposition to the development of certain specific and familial cancers is associated with the inheritance of a single mutated gene. In the best-characterized cases, this primary mutation is a loss of function mutation consistent with viability but resulting in neoplastic change consequent to the acquisition of a second somatic mutation at the same locus. Such genes are referred to as tumor-suppressor genes. Classical examples are the Rb-1 gene associated with the development of retinoblastoma and the p53 gene, which is associated with a wider range of neoplasms, including breast cancer. Other tumor-suppressor genes have been isolated which are associated with Wilms' tumor, neurofibromatosis, and inherited and sporadic forms of colorectal cancer. Some of these genes appear to act as negative regulators of mitotic cycle genes, and others may have different properties. The nature of these genes is discussed, as is the evidence for the involvement of tumor-suppressor genes in other inherited, and sporadic, forms of cancer. Some recent data on the Wilms' tumor gene, WT1, and on the involvement of the p53 gene in breast cancer are presented, and the importance of genomic imprinting in contributing to the excess of suppressor gene mutations in chromosomes of paternal origin is considered.