RB1 and breast cancer: Dysregulation of the cyclin D-cyclin-dependent kinase (CDK)4/6-retinoblastoma protein (RB) pathway causes uncontrolled proliferation of cells and has been implicated in the pathogenesis of HR-positive breast cancer [5], i.e., RB acts as a tumor suppressor by interacting with the E2F transcription factor and blocks its transcriptional activity.