This is a retrospective, multicenter, observational study in Japan by medical record review of advanced breast cancer (ABC) patients who have received palbociclib plus endocrine therapy (ET) as first line or second line setting. For the purposes of this study, analyses will be descriptive in nature. No formal hypothesis testing is planned.

Inclusion Criteria:

• Diagnosis of HR+/HER2- ABC
• Age of 20 years or older at diagnosis of ABC
• Patients that received palbociclib plus ET in the first line or second line
• Patients who meet the criteria regarding the medical record below:
• Patients with any medical records (regardless of palbociclib use) for more than 6 months from palbociclib initiation OR
• Patients with any medical records for less than 6 months from palbociclib initiation AND who had specific events (death, disease progression, or treatment discontinuation of palbociclib due to adverse events) in the available records

Exclusion Criteria:

• Patients who received chemotherapy as first line treatment.
• Patients who have previously participated or are participating in any interventional clinical trials that include investigational or marketed products.

(Interventional clinical trials for early breast cancer and later line after palbociclib will be acceptable, Besides, patients participating in other investigator-initiated research or noninterventional studies can be included as long as their standard of care is not altered by the study.)

This study uses secondary data of multiple sites by utilizing medical record review approach for those who have been diagnosed with ABC in Japan.

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.