RB1 and breast cancer: Synergistic efficacy of CDK4/6i and PARPi has been observed in both RB1‐proficient and ‐deficient BC cell lines, which is notable given CDK4/6i causes G1/S arrest in RB1‐proficent cells and prevent S phase entry, where PARPi exerts their cytotoxicity.216, 217, 218