To investigate the molecular mechanism that underlies the unique role of E2f1, we assessed the expression of known E2f targets as well as other genes that regulate the cell cycle and apoptosis. Numerous positive and negative cell cycle and apoptotic regulators were up-regulated in the Rb KO retina (Figure 1H). Among the E2f family, E2f1, E2f2, E2f3a, and E2f7 were induced following Rb loss, but E2f3b, E2f4, and E2f5 were unaffected. Consistent with the BrdU and terminal dUTP nick-end labelling (TUNEL) analyses, E2f1 deletion specifically reversed all these molecular defects, but E2f3 deletion had no effect (Figure 1H).