CDK4 and neoplasm: Cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) is a small molecule drug, taken orally, that dephosphorylates the retinoblastoma (Rb) protein by targeting CDK4 and 6.[4] The Rb protein dephosphorylation results in a cell cycle arrest blocking the progression of tumor cells from G1 to S phase.