text_1: Differentiated E2f-1, -2, -3 TKO cells in the intestine do not exhibit apoptosis.
text_2: In the established model of mammalian cell cycle control, the retinoblastoma protein (Rb) functions to restrict cells from entering S phase by binding and sequestering E2f activators (E2f1, E2f2 and E2f3), which are invariably portrayed as the ultimate effectors of a transcriptional program that commit cells to enter and progress through S phase.