CDK4 and breast cancer: Of note, another CDK4/6 inhibitor, ribociclib, was recently approved in the first-line treatment of postmenopausal, HR-positive/HER2-negative advanced breast cancer based on superior median PFS with ribociclib + letrozole (not reached) compared to placebo + letrozole (median PFS 14.7 months, hazard ratio 0.56 (95% CI 0.43–0.72, p < 0.0001) in the phase III MONALEESA-2 trial [6].