This study is designed to be a retrospective cohort study conducted at the Cancer Hospital Chinese Academy of Medical Sciences. The target populations of this study are patients with HR+/HER2- advanced breast cancer who received the initial endocrine therapy of palbociclib plus AI or fulvestrant monotherapy from August 1, 2018 to December 31, 2020. It is expected to enroll 600 subjects in this study. The subjects' data such as demographics and other baseline characteristics, medications, prognosis, will be collected retrospectively, and statistical analysis of data will be conducted to compare the effectiveness outcome measures.
