Treatment Monitoring of Patients Receiving CDK 4/6 Inhibitors for Hormone Receptor (HR) Positive, HER2 Negative Metastatic Breast Cancer (MBC) With or Without the Addition of DiviTum® Serum Thymidine Kinase 1 (TK1) Activity Testing

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 combination 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. Patients may also be eligible if: * Patients were treated with and progressed on prior endocrine therapy monotherapy in the metastatic setting, OR * Patients initiated endocrine therapy alone with ultimate intention to add CDK 4/6 inhibitor therapy, OR * Patients recurred on adjuvant endocrine therapy monotherapy and are scheduled to receive next line endocrine therapy combined with CDK 4/6 inhibitor. Patients may have also received CDK 4/6 inhibitor therapy in the adjuvant setting provided therapy completion occurred greater than 12 months prior to study enrollment. * Cohort 2 only: Currently receiving first-line combination therapy with any FDA-approved endocrine therapy plus any FDA-approved CDK 4/6 inhibitor (palbociclib, ribociclib, or abemaciclib). Changes in endocrine therapy or CDK 4/6 inhibitor agent during first-line combination therapy are permitted as long as change was not performed due to progressive disease. CDK 4/6 inhibitor must have been initiated within 24 months of study enrollment, and patient must have 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. Patients may also be eligible if they are receiving next line endocrine therapy plus CDK 4/6 inhibitor therapy following: * Progression on prior endocrine therapy monotherapy in the metastatic setting, OR * Recurrence on adjuvant endocrine therapy monotherapy. Patients may have also received CDK 4/6 inhibitor therapy in the adjuvant setting provided therapy completion occurred greater than 12 months prior to study enrollment. * Any prior therapy for early stage breast cancer is allowed, including endocrine therapy and chemotherapy. * 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). * Currently being treated at Siteman Cancer Center by a medical oncologist participating in this trial. 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. * Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the protocol assessments or analyses are eligible for this trial as determined by treating physician and with agreement by Principal Investigator * 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