CDK4

CDK4 is a cyclin-dependent serine/threonine kinase that partners with D-type cyclins to drive G1-phase progression before DNA synthesis[1]. Mechanistically, cyclin D-CDK4/6 phosphorylates RB, weakens RB-dependent transcriptional repression, and enables late-G1 signaling toward E2F-linked S-phase entry[2]. In cancer models, disruption of the CDK-RB1-E2F pathway supports malignant proliferation, and HR^+/HER2^- breast cancer provides a clinically established setting for CDK4/6 inhibition[3]. Compared with related isoforms, CDK4 and CDK6 share cell-cycle functions, but CDK4 is described as a prominent oncogenic driver in breast cancer, whereas CDK6 has a crucial role in hematopoietic stem-cell differentiation[3][4]. For experimental and translational applications, palbociclib, ribociclib, and abemaciclib inhibit CDK4/6, reduce RB phosphorylation, induce G1 arrest, and support endocrine-therapy combinations in HR^+/HER2^- advanced breast cancer studies[3][5][6][7].