E2F1 also known as RBP3 is a member of the E2F family of transcription factors. E2F1 plays a critical role in both normal cell cycle control and in the dysregulation of cell proliferation seen in various malignancies. E2F1 forms functional heterodimers with DP-1, and this complex drives the expression of genes that promote cell cycle progression and DNA replication. E2F1 is particularly important in regulating the transition from the G1 phase to the S phase, and E2F1 activity is tightly controlled by interactions with the retinoblastoma protein (Rb). Rb binding inhibits E2F-1 transcriptional activity, preventing the expression of cell cycle genes until appropriate signals trigger Rb inactivation, a process often disrupted in cancer.