RB1 and cancer: The most common cell cycle gene alterations include deletion of the p16 tumour suppressor gene (CDKN2A/B, deleted in approximately 20% of all cancer patients), deletions and mutations in the retinoblastoma protein (RB1, 7% of patients) and amplifications of the cyclin D1 (CCND1, 6% of all cancer patients, up to 30% in breast cancer5), E1 (CCNE1, 3.6%) and CDK4 (3.2%) genes6.