For several decades simian virus 40 (SV40) early region genes have been used as a means of generating immortalized human cell lines; however, the molecular mechanisms of this process have begun to be understood only recently. SV40-induced immortalization proceeds via two phases. In the first phase ("lifespan extension"), cells continue proliferating for a limited number of population doublings beyond the point at which normal cells undergo senescence. This is mainly due to the ability of SV40 large T antigen (LTAg) to bind to the protein products of the p53 and retinoblastoma (Rb) genes. The second phase ("immortalization") occurs in only a small minority of cells, and cell hybridization analyses indicate that this is a gene inactivation event. The gene or genes involved are currently unknown, but chromosomal localization data are accumulating which should make their cloning and characterization possible in the near future.