DiviTum®TKa: A Biomarker Assay for Efficacy in HR+ Metastatic Breast Cancer Patients

Inclusion Criteria: * Pre- or post-menopausal HR+ metastatic breast cancer patients who are: * Group 1: scheduled to initiate 1st line combination therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) and an FDA-approved endocrine therapy for pre- or post-menopausal HR+ metastatic breast cancer (mBC). * Group 2: scheduled to initiate second or later lines of therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) with or without an FDA-approved endocrine therapy for pre- or post-menopausal HR+ mBC. * Group 3: currently receiving 1st line therapy with an FDA-approved CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) and an FDA-approved endocrine therapy for pre- or post-menopausal HR+ mBC. * Group 4: scheduled to initiate, or are currently receiving, 1st or 2nd line therapy of an FDA approved endocrine therapy (single agent) for pre- or post-menopausal HR+ mBC. NOTE: Patients that meet eligibility criteria for any of the above groups that are already enrolled in an ongoing clinical trial are eligible for co-enrollment to this observational study. Exclusion Criteria: * Treatment including investigational agent or therapies * Early breast cancer diagnosis * Male breast cancer * Currently receiving treatment for other active malignancy at time of registration * EXCEPTIONS: Nonmelanoma skin cancer or carcinoma-in-situ (e.g. of cervix, prostate) * Inability to give written informed consent