This is an open-label, multicenter study in patients with metastatic breast cancer who are candidates for standard first line treatment with palbociclib or ribociclib plus endocrine therapy. To be eligible, patients must have received no prior chemotherapeutic or hormonal regimen for metastatic disease. However, patients may still be considered eligible if they have already started treatment with endocrine therapy (an aromatase inhibitor or fulvestrant) plus palbociclib or ribociclib for no longer than 4 weeks prior to study enrollment, as long as they meet all other eligibility criteria. Eligible patients must have had a diagnostic biopsy of the metastatic lesion no more than 4 months prior to study enrollment and with sufficient tissue to complete the proposed biomarker analysis. Patients who develop disease progression within the first 12 months of starting palbociclib or ribociclib plus endocrine therapy will be eligible for an optional additional tissue biopsy at time of disease progression to repeat the analysis at time of disease progression and obtain real-time (10-14-day turn-around) multi-omic data produced under College of American Pathologist (CAP)/Clinical Laboratory Improvement Amendments (CLIA) development and/or compliant practices.

Inclusion Criteria:

• Patients must have histologically or cytologically proven diagnosis of adenocarcinoma of the breast with evidence of locoregionally recurrent or metastatic disease.

• Tumors must be estrogen and/or progesterone receptor positive according to ASCO/CAP 2010 guidelines as either ER or PR ≥ 1% positive nuclear staining by immunohistochemistry based on local laboratory results.

• Tumors must be HER2 negative as defined according to ASCO/CAP 2013, as HER2 0 - 1+ by IHC or non-amplified FISH or CISH. If HER2 IHC is 2+, FISH/CISH must be performed and must not be positive (HER2/CEP17 ratio must be < 2, and HER2 copy number < 6 signals/cell), but otherwise FISH/CISH is not required if IHC is 0 or 1+ by institutional standards.

• Must be candidates to receive endocrine therapy and palbociclib or ribociclib as first-line treatment for their advanced disease. Patients will be considered eligible for study enrollment if they have started on treatment with a standard dose and schedule of palbociclib or ribociclib and endocrine therapy (aromatase inhibitor or fulvestrant) as long as they have not started palbociclib or ribociclib treatment for longer than 4 weeks from time of study enrollment, have sufficient tissue to perform the proposed tissue analysis and must meet all other eligibility criteria. Endocrine therapy can be initiated up to 4 weeks prior to starting palbociclib or ribociclib.

• Patients must have measurable disease by RECIST v.1.1 or bone disease as their only site of disease (with bone lesions confirmed by CT, MRI or bone X-ray).

• No prior treatment with chemotherapy for a diagnosis of locoregionally recurrent or metastatic breast cancer is allowed.

• Be ≥ 18 years of age

• Have an ECOG score of 0-1

• Postmenopausal women defined as women with:

  • Prior bilateral surgical oophorectomy, or
  • Medically confirmed post-menopausal status defined as spontaneous cessation of regular menses for at least 12 consecutive months or follicle-stimulating hormone (FSH), luteinizing hormone (LH) and estradiol blood levels in their respective postmenopausal ranges.

• Premenopausal women will be considered eligible for study participation if they are receiving medical ovarian suppression with luteinizing hormone-releasing hormone (LHRH) agonists with documented estradiol blood levels in their respective postmenopausal ranges.

• Archive tumor tissue (obtained from a biopsy or surgical resection of a metastatic lesion done within 4 months from study enrollment) availability is required for patient participation. If the available tissue is insufficient for the required baseline analysis, the patients are given the option to repeat the biopsy for the purpose of study participation as long as they have not already started palbociclib or ribociclib.

• Understand and provide written informed consent prior to initiation of any study-specific procedures.

Exclusion Criteria:

• Lack of archive tumor tissue from a biopsy or surgical resection of a metastatic lesion done within 4 months of study enrollment. Patients will be given an option to have a repeated biopsy of a metastatic lesion if they had a diagnostic tumor biopsy intended for use in the current study that was performed more than 4 months prior to analysis, or there is insufficient tissue from the initial biopsy to complete the analysis, as long as they have not started treatment with a CDK 4/6 inhibitor. Otherwise, the patient will be excluded from the study participation.
• Have symptomatic CNS metastasis. Patients with a history of CNS metastases who have been treated with whole brain irradiation must be stable without symptoms for 4 weeks after completion of treatment, with image documentation required, and must be either off steroids or on a stable does of steroids for ≥ 4 weeks prior to enrollment.
• Have uncontrolled concurrent illness including, but not limited to, ongoing or active serious infection, symptomatic congestive heart failure, unstable angina pectoris, unstable cardiac arrhythmias, psychiatric illness, or situations that would limit compliance with the study requirements or ability to willingly give written informed consent.

Open-label