RB1 and cancer: For example, CDK4/6 inhibitors were initially envisioned to (a) protect bone marrow cells (not necessarily all types of normal cells) (b) from chemotherapy with S-phase-specific drugs such as 5-fluorouracil and carboplatin (c) in patients with Rb1-negative cancers (resistant to CDK4/6 inhibitors) [1, 2, 45].