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.