E2F1 and neoplasm: Studies in breast, lung, head and neck, esophagus, and colon mantle cell lymphoma found that E2F-1 overexpression is associated with increased proliferation and aneuploidy, and that cyclin D1 acts as a positive regulator of the transition from G1-phase to S phase by association with and activation of CDKs, leading to uncontrolled growth of tumor cells [21,22].