Comparative Effectiveness of PAL Based-therapy as Initial ET Versus PAL Based-therapy After ChT for HR+/HER2- ABC

Inclusion Criteria: 1. Age ≥ 18 years, female (retrospective part); 2. Histologically or cytologically confirmed recurrent or metastatic HR+/HER2- breast cancer, if there was metastatic pathology, the histological pathology of metastases should prevail (retrospective part); 3. Received palbociclib in combination with endocrine therapy as initial therapy in the first line and received at least 1 cycle; or received systemic chemotherapy in the first line followed by sequential palbociclib in combination with endocrine therapy and received at least 1 cycle (retrospective part); 4. The follow-up time was not less than 3 months after the initiation of palbociclib in combination with endocrine therapy (retrospective part); 5. Age ≥ 18 years, female (prospective part); 6. Histologically or cytologically confirmed recurrent or metastatic HR+/HER2- breast cancer, if there is metastatic pathology, the histological pathology of metastases should prevail (prospective part); 7. ECOG PS ≤ 2, and expected survival of more than half a year and no life-threatening visceral metastasis (prospective part); 8. Measurable disease (prospective part); 9. No or up to 1 regimen of chemotherapy for advanced breast cancer (prospective part); 10. The patient has good compliance, and needs to receive the treatment with the palbociclib-based therapy according to the condition (prospective part); 11. Normal function of major organs and hematopoietic function, i.e. meeting the following criteria: Blood cell count criteria should be in accordance with: (no blood transfusion and blood products, no G-CSF and other hematopoietic stimulating factors for correction in recent 14 days): Hemoglobin (HB) ≥ 80 g/L; Neutrophils (ANC) ≥ 1.5 × 10\^9/L; Platelet (PLT) ≥ 75 × 10\^9/L. Liver function, renal function and electrolytes meeting the following criteria: Total bilirubin (TBIL) \< 1.5 upper limit of normal (ULN), ≤ 3 ULN for patients with liver metastases; Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 ULN, and ≤ 5 ULN for patients with liver metastases; Serum Cr ≤ 1.5 ULN or endogenous creatinine clearance ≥ 60 ml/min (Cockcroft-Gault formula); Electrolytes: magnesium ≥ lower limit of normal (LLN) (prospective part); 12. The patient is compliant and voluntarily accepts the treatment, follow-up visit, laboratory examination and other study procedures according to the doctor's prescription (prospective part). Exclusion Criteria: 1. ≥ 2 prior lines of chemotherapy for advanced breast cancer (retrospective part); 2. Treatment with other CDK4/6 inhibitors (abemaciclib or ribociclib) or endocrine therapy only (retrospective part); 3. Life-threatening visceral metastases, and lesions not evaluable for response (prospective part); 4. Neutrophils \< 1.5 × 10\^9/L; platelets \< 100 × 10\^9/L; not improved after medication (prospective part); 5. Total bilirubin ≥ 1.5 times the upper limit of normal, AST and ALT ≥ 2 times the upper limit of normal; serum Cr ≥ 1.5 times the upper limit of normal (prospective part); 6. Have central nervous system or meningeal invasion (prospective part); 7. Women who are pregnant, lactating, or planning to have children (prospective part); 8. Received ≥ 2 prior lines of chemotherapy for advanced breast cancer (prospective part); 9. Treatment with other CDK4/6 inhibitors (abemaciclib or ribociclib) or endocrine therapy only (prospective part); 10. Concomitant poorly controlled serious illness, major surgery, or history of other malignancy within 5 years (prospective part); 11. Other conditions that the investigator considers inappropriate for inclusion (prospective part).