BettER: Biomarker Driven Early Therapeutic Selection in Patients With HR+ HER2- Metastatic or Unresectable Breast Cancer

Inclusion Criteria - Patients * Diagnosis of metastatic or advanced unresectable invasive breast cancer that is hormone receptor-positive (HR+) and HER2-negative. * Planned to initiate standard of care first-line therapy with FDA-approved endocrine therapy plus CDK4/6 inhibitor for the stated diagnosis at the time of study enrollment. Ribociclib is the preferred CDK4/6 inhibitor. In the event this drug cannot be obtained due to insurance authorization or if there are specific side effect profile concerns from the treating physician, an alternative CDK4/6 inhibitor is allowed. * Any prior therapy for early stage breast cancer is allowed, including endocrine therapy and chemotherapy. * Prior receipt of adjuvant CDK 4/6 inhibitor therapy is permitted provided therapy completion occurred \> 12 months prior to study enrollment. * Presence of RECIST-evaluable disease. Patients with bone-only disease are eligible. * At least 18 years of age. * ECOG performance status ≤ 2 * Post-menopausal status, defined as one of the following: * Age ≥ 60 years * Age \< 60 with intact uterus and amenorrhea for 12 consecutive months or more * Status post bilateral oophorectomy, total hysterectomy * Pre- or peri-menopausal with suppressed ovarian function by use of GnRH agonist/antagonist or surgical bilateral oophorectomy * Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable). Exclusion Criteria - Patients * Receipt of any prior cytotoxic chemotherapy line for metastatic disease. There will be no limit to chemotherapy use in the neoadjuvant or adjuvant setting. * Patients with a prior or concurrent malignancy are excluded unless that malignancy's natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial. * Concurrent participation in any investigational therapeutic trial for treatment of metastatic breast cancer. Eligibility Criteria - Physicians * Medical Oncologist at Siteman Cancer Center. * Treating patients with metastatic or advanced unresectable breast cancer. * Willing to complete Physician Surveys during participation.