The role tumor suppressors p53 and retinoblastoma (RB) play in the transformation process has become central to understanding the pathogenesis of DNA tumor viruses. The two oncoproteins of human papillomavirus (HPV)-16, E6 and E7, bind to p53 and RB, respectively, thus inactivating the function of these tumor suppressor genes. Immortalization of primary human foreskin epithelial cells by HPV requires expression of the E7 protein, and the E6 protein greatly enhances the immortalization frequency. Two of three cell lines immortalized by the HPV-16 E7 oncoprotein expressed wild-type p53 and only one of the three cell lines had acquired a p53 mutation and loss of heterozygosity at 17p during the immortalization process. All three E7-immortalized lines contained higher steady-state levels of p53 protein. Mutation of the p53 gene is not required for immortalization in the absence of the HPV-16 E6 inactivation of the p53 protein, and 16E7 expression leads to the stabilization of wild-type p53.