This is a prospective, single-arm, multi-center observational non-interventional study (NIS) in Germany and Austria.

Patients diagnosed with HR+/HER2- locally advanced or metastatic breast cancer indicated by their treating physicians for first line endocrine-based palbociclib combination therapy and who meet eligibility criteria will be invited to participate in this study. The key objectives of this study are to describe clinical, scientific and patient reported outcomes for patients with HR+/HER2- locally advanced or metastatic breast cancer initiating treatment with first line endocrine-based palbociclib combination therapy in the real-world setting in Germany and Austria. Patient characteristics, real-world treatment patterns, treatment sequences and reasons for the physician's treatment decisions will be collected. Additional real-world research questions are to explore patient-focused parameters such as longitudinal follow-up data on patient-reported outcomes beyond disease progression and by treatment sequence or to analyze the time from the start of first line treatment to the first administered palliative chemotherapy. Clinical outcome by treatment sequences will be described. Routinely assessed biomarkers and diagnostic procedures applied for treatment sequence decisions will be collected.

Patients must meet all of the following inclusion criteria to be eligible for inclusion in the study:

1. Evidence of a personally signed and dated informed consent document indicating that the patient has been informed of all pertinent aspects of the study.
2. Diagnosis of HR+/HER2- locally advanced, inoperable or metastatic breast cancer.
3. Physician has determined that first-line treatment with palbociclib (i) in combination with an aromatase inhibitor, or (ii) in combination with fulvestrant in women who received prior endocrine therapy as per current local product label is indicated. In pre- or perimenopausal women, the endocrine therapy should be combined with a luteinizing hormone-releasing hormone (LHRH) agonist.
4. Patients who in the opinion of the investigator are willing and able to comply with regular clinic visits as per local standard of care practice at the study site.
5. Age of 18 years or older.

Patients meeting any of the following criteria will not be included in the study:

1. Any contraindication as per current local product label.
2. Prior systemic antineoplastic treatment for advanced disease. Exception: Start of first line treatment with palbociclib in combination with aromatase inhibitor or fulvestrant as per current local product label is allowed up to 4 weeks prior to inclusion.
3. Patients currently participating in any interventional clinical trial that includes investigational or marketed products at the time of enrollment. Note: A concomitant participation in other non-interventional/observational studies, registries and translational research networks (e.g., PRAEGNANT, OPAL) or chart reviews is allowed.
4. Patients who are unable to understand the nature of the study or are unwilling to sign an informed consent.

Patient eligibility should be reviewed, documented, and confirmed by an appropriately qualified member of the investigator's study team before patients are enrolled in the study.

Patients diagnosed with HR+/HER2- locally advanced or metastatic breast cancer indicated by their treating physicians for first line endocrine-based palbociclib combination therapy and who meet eligibility criteria will be invited to participate in this study.

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.