Phase IIIb Study of Ribociclib + ET in Early Breast Cancer

Key Inclusion criteria: * Participant is an adult, male or female ≥ 18 years of age at the time of informed consent form signature (IC). * Participant has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer (BC) based on the most recently analyzed tissue sample tested by a local laboratory prior to enrollment. * Participant has HER2- BC defined as a negative in situ hybridization test or an immunohistochemistry (IHC) status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test is required by local laboratory testing based on the most recently analyzed tissue sample. * Participants may have already received any standard neoadjuvant and/or adjuvant ET, including tamoxifen or toremifene at the time of informed consent signature, but enrollment should occur within 36 months of prior ET start date and participants should have at least 3 years remaining of endocrine adjuvant therapy. * For participants with prior ET treatment \> 12 months, restaging is highly recommended (unless contradictory to local regulations) to rule out disease recurrence prior to enrollment. * The number of participants with prior ET between 12 and 36 months will be capped at 30%. The cap will not apply to Black or African American participants. * Participant has no contraindication to receive adjuvant ET in the study. * Participant after surgical resection where tumor was removed comp…