This is a retrospective, observational study that will document the treatment and monitoring patterns and clinical outcomes of patients diagnosed with HR+/HER2- a/mBC who received Palbociclib combination therapy with aromatase inhibitors or fulvestrant in the a/mBC community oncology setting.

Inclusion Criteria:

Patients who are represented in the Vector Oncology Data Warehouse who meet all of the following criteria will be eligible for inclusion in the study.

• Female.
• Diagnosis of female breast cancer with evidence of metastatic disease or advanced disease.
• Diagnosis (confirmed by clinical review) of advanced or metastatic breast cancer at any time, defined as breast cancer at stage IIIb, stage IIIc, or stage IV or identified as having distant metastasis.
• Indication in the record of ER or PR positive disease, or absence of any indication of ER and PR negative disease (ie, patients are eligible without affirmative indication of ER/PR+ status as long as ER/PR- indication is not present).
• Receipt of a palbociclib plus aromatase inhibitor (AI) or palbociclib plus fulvestrant regimen on 2/3/2015 or later in the advanced and metastatic setting. Data collection will prioritize accrual of patients with 1L use of palbociclib plus AI and with 2L use of palbociclib plus fulvestrant before collecting other use of palbociclib users.
• Age greater than or equal to 18 years at A/MBC diagnosis.

Exclusion Criteria:

-There are no exclusion criteria for this study.

The study will include adult patients aged 18 years and older, diagnosed with HR+/HER2- a/mBC who received Palbociclib combination therapy with AI or fulvestrant on or after 2/3/2015.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.