• age ≥18 years old; Invasive breast cancer with metastatic disease confirmed by histological or cytological examination; Patients without pathologically or cytologically confirmed metastatic disease should have clear evidence of metastasis by physical examination or radiological studies;

• The most recent pathological report of biopsy confirmed HR low expression and HER2 negative.

∙ HR low expression was defined as 1-50% ER expression by immunohistochemistry (IHC); Or ER\<1% and PR≥1%; Patients with ER expression of 1-10% or ER-/ PR-positive patients were eligible for inclusion after careful evaluation by the investigator, and those with a small tumor burden and candidates for endocrine therapy were eligible.

‣ HER2-negative definition: IHC 0 or 1+; If the IHC was 2+, it was confirmed negative by fluorescence in situ hybridization (FISH).

• at least one measurable lesion;

• No previous salvage chemotherapy for metastatic disease was required, and first-line endocrine therapy was allowed;

• no previous CDK4/6 inhibitor; For adjuvant CDK4/6i treatment, recurrence and metastasis were required more than 1 year after drug withdrawal.

• Eastern Cooperative Oncology Group performance status (ECOG PS) 0-1, life expectancy is more than 12 weeks;

• Adequate function of major organs.

• All adverse events recovered to grade 1 or less before enrollment (NCI CTCAE version 5.0);

• patients without major organ dysfunction and heart disease;

⁃ Women and men of childbearing potential must agree to use appropriate contraception before and during study participation.