Inclusion Criteria - Patients:  Diagnosis of metastatic or advanced resectable invasive breast cancer that is hormone receptor-positive (HR+) and HER2-negative. Tumor assessment by radiographic imaging will be performed within 4 weeks of baseline study visit. Cohort 1 only: Scheduled to initiate standard of care first-line therapy with any FDA-approved endocrine therapy plus any FDA-approved CDK 4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) for the stated diagnosis at the time of study enrollment. Type of endocrine therapy and CDK 4/6 inhibitor will be documented. Cohort 2 only: Currently receiving first-line therapy with any FDA-approved endocrine therapy plus any FDA-approved CDK 4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) for no longer than 24 months with at least stable disease (no progression) on such therapy for a minimum of 12 weeks prior to enrollment as determined by radiographic studies as deemed appropriate by the treating physician. Type of endocrine therapy and CDK 4/6 inhibitor will be documented. Any prior therapy for early stage breast cancer is allowed, including endocrine therapy and chemotherapy. Cohorts 1 and 2: Prior receipt of adjuvant CDK 4/6 inhibitor therapy is permitted provided therapy completion occurred > 12 months prior to study enrollment. At least 18 years of age. Life expectancy > 6 months.  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 limited to chemotherapy use in the neoadjuvant or adjuvant setting. A history of other malignancy with the exception of malignancies for which all treatment was completed at least 2 years before registration and the patient has no evidence of disease. 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 invasive breast cancer Willing to complete Study Care Plans on a serial basis during participation