Figure 1.

Schematic overview of cell cycle regulation, with an emphasis on radiotherapy-induced pathways and CDK/cyclin regulation. In M phase and in G2 resting phase, cancer cells are respectively very sensitive and moderately sensitive to radiation injury, whereas in G1 phase and in S phase, cancer cells are moderately resistant to radiation injury. Irradiation induces G1 and G2 cell cycle checkpoint activation and DNA repair. Most cancer cells are defective in G1 checkpoint, commonly due to the mutations/alterations of the key regulators of the G1 checkpoint, but contain a functional G2 checkpoint.