Inclusion Criteria:  Chinese women ≥18 years old; Diagnosed with locally advanced (stage IIIb/IIIc) or metastatic (stage IV) breast cancer; The molecular classification is pathologically confirmed as HR+/HER2-; Palbociclib plus AI or fulvestrant monotherapy as initial endocrine therapy for advanced breast cancer for at least 1 cycle; The follow-up time should be not less than 3 months after the start of palbociclib plus AI therapy or fulvestrant monotherapy.  Exclusion Criteria:  1. Subjects who have previously received chemotherapy, other CDK4/6 inhibitors or other endocrine monotherapy for advanced breast cancer.