Breast cancer is the most common cancer of women in Western countries. Various genetic alterations have been implicated in its development. Two tumor suppressor genes, the retinoblastoma susceptibility gene (RB) and the gene encoding the p53 protein, are frequently found to be deleted or mutated in breast cancer cell lines and primary tumor samples. Breast carcinoma cell lines MDA-MB468 and BT549 both harbor partial RB gene deletions as well as point mutations of their p53 genes, thus providing an excellent model system for testing the roles played by these two genes in the oncogenesis of breast cancer. Single copies of wild-type RB or p53 were delivered to these cells by retrovirus-mediated gene transfer. Restoration of RB expression in cells reduced their ability to grow in soft agar and their tumorigenicity in nude mice, although no significant alteration of growth rate in culture could be detected. Introduction of wild-type p53 into these cells exerted a similar effect on the neoplastic phenotypes. This effect occurred even in the presence of their endogenous mutated p53 alleles, which argues for the phenotypical dominance of the wild-type p53 over mutated p53 during human oncogenesis. The ability of RB and p53 genes to suppress the tumorigenicity of breast carcinoma cells provides functional evidence that deletion or mutational inactivation of tumor suppressor genes represents an important step in the genesis of breast cancer.