A schematic representation of RB/p53 interactions to regulate cell cycle and apoptosis. Cell cycle transition from G1-S phase is mediated by RB interactions with the E2F transcription factor family, which is considered an important regulator of the cell cycle. Growth factors lead to the phosphorylation of RB in late G1 phase by cdk/cyclin. This is followed by the release of E2F, allowing transcriptional activation of E2F target genes, which promotes S-phase entry and cell proliferation. HPV E7 and Simian Virus 40 (SV40) promote the release of E2F from RB, whereas HPV E6 and the dominant negative, DN-p53 inhibit p53 activity leading to cell proliferation.