Study of the Molecular Features of Postmenopausal Women With HR+ HER2-negative aBC on First-line Treatment With Ribociclib and Letrozole and, in Patients With a PIK3CA Mutation, on Second-line Treatment With Alpelisib Plus Fulvestrant

CORE PHASE Inclusion Criteria: * Patient has an advanced (locoregionally recurrent or metastatic) breast cancer in first line treatment (treatment naïve for the advanced setting). * Patient is in post-menopause, defined by one of the following: * Prior bilateral oophorectomy * Age ≥60 * Age \<60 and amenorrhea for 12 or more months (in the absence of chemotherapy, tamoxifen, toremifen, or ovarian suppression) and FSH and estradiol in the postmenopausal range per local normal range * Patient has a histologically and/or cytologically confirmed diagnosis of estrogenreceptor positive and/or progesterone receptor positive breast cancer by local laboratory. * Patient has an HER2-negative breast cancer defined as a negative in situ hybridization test or an 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. * Patient is willing to undergo blood and tumor sample collection for the biological assessments/objectives as scheduled in the protocol. CORE PHASE Exclusion Criteria: * Patient who received prior treatment with any CDK4/6 inhibitor. * Patient who received any prior systemic hormonal therapy or chemotherapy for advanced breast cancer. Note: Patients who received neo/adjuvant therapy for breast cancer are eligible. If the prior neo/adjuvant therapy included letrozole or anastrozole, the disease-free interval must be greater than 12 months from the completion of treatment until study ent…