“…It has been suggested that in late G 1 , genetically controlled event triggers the initiation of DNA synthesis, sometimes referred to as a transition probability event; the probability of passing through this transition is high in the gut, and thus the cells cycle rapidly. 1 In the gastrointestinal system, cells eventually leave the cell cycle and diVerentiate, but in some tissues they may enter a resting phase, known as G 0 , although there is little evidence of a G 0 phase in the gut. DiVerentiated cells are generally believed to lose their proliferative capacity, but in extreme conditions, they can be persuaded to enter the cell cycle again by expression of, for example, SV40T antigen.…”