HARMONIA is an international, multicenter, randomized, open-label and phase III study. The primary objective of this study is to demonstrate that the combination of ribociclib with endocrine therapy (letrozole or fulvestrant) is superior to palbociclib with endocrine therapy (letrozole or fulvestrant) in prolonging progression-free survival in patients with advanced HR+/HER2- and HER2-E breast cancer. The study will enroll approximately 456 patients with HER2-E disease from approximately 95 sites worldwide. In addition, the HARMONIA trial will include an exploratory cohort of patients with HR+/HER2- and Basal-like disease treated with paclitaxel +/- Tislelizumab. This cohort does not have a predefined sample size and the objective is only exploratory, given the suggested lack of efficacy of the combinations of hormone therapy and CDK4/6 inhibitors in this subgroup of patients. Enrolment into the basal-like cohort will stop once the HER2-E disease cohort is fully enrolled.