Transcription factor E2F1 (E2F1), also known as RBBP3, is a transcription activator that binds DNA cooperatively with dp proteins through the E2 recognition site, 5'-TTTC[CG]CGC-3' found in the promoter region of a number of genes whose products are involved in cell cycle regulation or in DNA replication. The DRTF1/E2F complex functions in the control of cell-cycle progression from G1 to S phase. E2F-1 binds preferentially RB1 protein, in a cell-cycle dependent manner. It can mediate both cell proliferation and p53-dependent apoptosis. The calculated molecular weight of E2F1 is 47 kDa, but the sumoylated E2F1 is bout 55-60 kDa.