The E2F transcription factors are essential for regulation of the cell cycle. Physiological E2F is a heterodimer composed of an E2F subunit together with a DP subunit. Six members of the E2F family have been identified, and each E2F subunit has a DNA binding and a dimerization domain. E2F-1 to -5 activate transcription. E2F-1 to -3 bind pRb, and E2F-4 and -5 bind p107 or p130, and these interactions are under cell cycle control. E2F-1 has oncogenic properties in vivo and in vitro. E2F-1 can induce apoptosis through p53-dependent and -independent mechanisms. E2F-1 is stress-responsive, and is regulated by a PI3-kinase-like kinase family such as the ATM/ATR kinases.