RB1 and cancer: This is considered to be due to alterations in expression of molecules such as p53, p21WAF/CIP1, pRB, p27, and cyclins A, B, and E. Similar effects can be seen following exposure of cells to ionizing radiation, which can cause arrest at the G1 and G2/M checkpoints, as well as S-phase delay, and apoptosis in certain cancer cell lines [48].