The E2F1-3 transcription factors are essential for cellular proliferation

The retinoblastoma tumour suppressor (Rb) pathway is believed to have a cri tical role in the control of cellular proliferation by regulating E2F activ ities(1,2). E2F1, E2F2 and E2F3 belong to a subclass of E2F factors thought to act as transcriptional activators important for progression through the G1/S transition(3). Here we show, by taking a conditional gene targeting a pproach, that the combined loss of these three E2F factors severely affects E2F target expression and completely abolishes the ability of mouse embryo nic fibroblasts to enter S phase, progress through mitosis and proliferate. Loss of E2F function results in an elevation of p21(Cip1) protein, leading to a decrease in cyclin-dependent kinase activity and Rb phosphorylation. These findings suggest a function for this subclass of E2F transcriptional activators in a positive feedback loop, through down-modulation of p21(Cip1 ), that leads to the inactivation of Rb-dependent repression and S phase en try. By targeting the entire subclass of E2F transcriptional activators we provide direct genetic evidence for their essential role in cell cycle prog ression, proliferation and development.