RB1 and breast cancer: Since CDK 4/6 can promote cell‐cycle entry from the G1 phase to the S phase by phosphorylating Rb protein, palbociclib inhibits CDK 4/CDK 6 therefore leading to tumor growth limitation.5, 31 In particular, the CDK 4/6 inhibitor palbociclib has shown high activity in ER‐positive and HER2‐negative advanced breast cancer, and it may result from the inhibition of CDK 4/6.