Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of 'recessive cancer genes', the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumour growth by normal cell products. Such tumour suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes. Consideration of both kinds of genes may reveal common or interrelated functional properties.