CDK4 and breast carcinoma: A total of 119 patients with metastatic HR+/HER2-negative breast cancer (patients could have received up to one line of chemotherapy) were included in the study and were randomized into two arms: the first received (switch) ET combined with ribociclib, and the other arm (switch) ET combined with placebo (60 and 59 patients, respectively); the initial CDK4/6 inhibitor used in the prior line was palbociclib in the majority of patients.