RB1 and breast cancer: Dysregulation of the CDK4/6 pathway, occasioning unchecked cell-cycle progression and proliferation, has been implicated in breast cancer (BC) via various mechanisms, including amplification of cyclin D1 (2), gain of CDK4, low expression of p18/high expression of RB1 (3) and inactivation of p16, the tumor suppressor protein and negative moderator of the cyclin D-CDK4/6 complex (4).