A Study to Understand About the Study Medicine Palbociclib in Breast Cancer Patients After it is Out in the Japanese Market

The purpose of the study is to: * compare the chances of getting interstitial lung disease (ILD) in new users of palbociclib plus fulvestrant to new users of fulvestrant alone (using data from before palbociclib was out in the market) * look into the factors that can cause ILD in new users of palbociclib. This study uses patient data without giving out any personal information of the patient. This data is taken from a hospital-based claims database of the MDV database, that includes data from more than 400 hospitals in Japan. This study will use data of patients who are: * diagnosed with breast cancer that include newly treated with palbociclib and/or other endocrine-based therapies for inoperable or recurrent breast cancer between 25 November 2011 and 30 November 2024.

Inclusion Criteria: * Having prescription records of drugs for endocrine therapy (aromatase inhibitors, SERDs, SERMs, and CDK4/6 inhibitors) from 25 November 2011 to 31 December 2024. * Having any breast cancer records with definitive diagnosis in the same month as the first prescription date. * Having any records in MDV database on or before 6 months prior to the first prescription date (\