Phase IIIb Study of Ribociclib + ET in Early Breast Cancer

The study consists of Screening, Treatment, and Follow-up periods. * Treatment Period: all participants who complete screening will receive ribociclib 400 mg orally once daily on days 1 to 21 of a 28-day cycle, in combination with daily ET for 36 months (approximately 39 cycles) from the date of first dose. The Treatment Period starts when the patient receives their first dose of ribociclib and ends at the time of the 30-day Safety Follow-up. All treated participants should have a Safety Follow-up call conducted 30 days after the last dose of study treatment. * Follow-up period: participants will be followed from 30 days after study treatment (i.e., ribociclib) completion/discontinuation (i.e. 30-day Safety Follow-up) until death, withdrawal of consent, lost to follow-up, or until 48 months after the last participant has received their first dose of study treatment (i.e. End of Study \[EOS\]), whichever occurs first.