Figure 2.

Extending the restriction point. A. Cell cycle progression is dependent on CYCLIN-CDK kinase activity. B. Mitogen starvation results in cell cycle arrest in G₁ through degradation or suppression of CYCLIN D and CKI-mediated inhibition of CYCLIN E-CDK2. C. Unscheduled passage through the G₁ restriction point in the absence of mitogens (e.g., through RB loss) results in cell cycle arrest in G₂.