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Cell cycle progression is a tightly controlled process. To initiate cell division, mitogens trigger a number of early signals that promote the G₀–G₁ transition by inducing cell growth and the activation of G₁ cyclins. Activation of cyclin E/cdk2 (cyclin-dependent kinase 2) at the end of G₁ is then required to trigger DNA synthesis (S phase entry). Among the early signals induced by mitogens, activation of PI3K (phosphoinositide 3-kinase) appears essential to induce cell cycle entry, as it regulates cell growth signalling pathways, which in turn determine the rate of cell cycle progression. Another mechanisms by which PI3K and its downstream effector protein kinase B regulate cell cycle entry is by inactivation of the FOXO (Forkhead Box, subgroup O) transcription factors, which induce expression of quiescence genes such as those encoding p27^kip, p130 and cyclin G2. PI3K/FOXO then work as a complementary switch …