Final answer:
E2F is a transcription factor involved in the regulation of the cell cycle, specifically in the transition from the G1 to the S phase.
Step-by-step explanation:
The protein E2F can be classified as a transcription factor. It plays a crucial role in the cell cycle, particularly during the transition from the G1 to the S phase. The retinoblastoma protein (Rb) monitors cell size and, in its dephosphorylated state, binds to E2F, preventing it from activating genes necessary for this cell cycle progression. Once the cell has grown and Rb becomes phosphorylated, it releases E2F, which then can activate genes leading to DNA replication and cell cycle progression. This action permits the cell to proceed past the G1/S transition, a critical checkpoint in the cell cycle.