Effects of Ribociclib and Palbociclib on Tumor and Blood Characteristics in Patients With Metastatic Breast Cancer

Inclusion Criteria: * Patients with histological confirmed breast adenocarcinoma who meet the following criteria: * Age: \>= 18 years and are post-menopausal * Estrogen receptor positive (ER+) and human epidermal growth factor receptor 2 (HER2) negative breast cancer patients who prospectively may undergo evaluation for Cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i), ribociclib or palbociclib, as part of first-line therapy. Retrospective cohorts will be built to include either ribociclib or palbociclib treated patients with known clinical outcome * During routine standard of care procedure, tumor that is accessible for ultrasound guided biopsy (from breast, lymph node, subcutaneous tumor, or selected liver metastasis per treating physician's discretion) or skin punch biopsy (for dermal metastasis) will be collected * Available tumor tissue or planning on biopsy prior to initiation of CDK4/6i treatment * Available tumor tissue or planning on biopsy at time of progression, prior to initiating subsequent therapy * Willing to provide consent for extra tissue and blood samples Exclusion Criteria: * Patient received prior treatment with any CDK4/6 inhibitor * Prior treatment with any chemotherapy for metastatic disease * Patient is cognitively impaired * Lung or bone metastasis only (not accessible by ultrasound guided biopsy) * Patients with central nervous system (CNS) involvement unless they meet ALL the following criteria: * Untreated brain metastases (e.g., lesions…