Figure 7
Model of the G1 phase of the cell cycle showing the Restriction point and postulated TS activation window. In the absence of mitogenic signals, cells are in a resting (G0) state. Upon stimulation with growth factors, cells enter G1 and cyclin D levels increase. As G1 progresses, cyclin D-CDK4/6 complexes phosphorylate pRb leading to the release of E2F/Dp1 complexes. E2Fs then activate the transcription of a series of S-phase genes and cyclin E. Once cyclin E is synthesised, it associates with CDK2 and drives S-phase entry. The TS activation window is proposed to coincide with the period that CDK4 is active.
