Studies on human papillomavirus type 16 have demonstrated that the product of the early gene, E7, plays a key role in the immortalization and malignant transformation of the host cell. Several of the biological activities of HPV16 E7 are mediated by inactivation of the members of the pocket protein family, pRb, p107 and p130. In this study, we have characterized the in vitro properties of ®ve E7 proteins from benign and malignant HPV types (10, 32, 48, 54, 77). We show that these E7 proteins associate with pRb and p107 with di erent e ciencies. All E7s increased the proliferative rate of immortalized rodent ®broblasts cultured in 10% calf serum containing medium. This property is completely independent of their ability to associate with the pocket proteins. Furthermore, all E7s, except HPV10 E7, stimulate G1/S progression and activated the cyclin E and cyclin A promoter in the absence of growth factors. This activity also does not correlate with the E7-e ciency of binding the pocket proteins. Together these data provide evidence that di erent E7s alter the regulation of the cell cycle by diverse mechanism(s). Finally, this comparative analysis of the di erent E7 proteins demonstrates that the oncogenicity of a HPV type is not determined by the ability of E7 to associate with the pocket proteins. Oncogene (2000) 19, 821 ± 826.