Study of hereditary cancer in humans has revealed new mechanisms in carcinogenesis. In particular, a new class of cancer gene, recessive in oncogenesis, accounts for dominantly transmitted predisposition to some cancers, and may play a primary role in the nonhereditary forms of most cancers. Comparison of polymorphic markers in lymphocytes and tumors has permitted the in vivo observation of somatic events that lead to genetic recombination. The tissue specificities of these recessive cancer genes suggest that their normal alleles, like those of oncogenes, play important roles in normal development.