CDK4 and breast carcinoma: Many cancers are characterised by abnormal overproduction of cyclin D1.8, 9 As Cdk4 inhibitors target a pathway that links pRb, p16INK4A, cyclin D1 and Cdk4, it makes inhibition of Cdk4-cyclin D1 enzyme a crucially important target for cancer chemotherapy.10, 11, 12, 13 However, Rb mutations, consistent with loss of Rb function, have been identified in a wide spectrum of tumours including osteosarcomas, small-cell lung carcinomas, breast carcinomas and others, and the Cdk4 inhibitors cannot inhibit such pathway involving Rb-mutated tumours.