To evaluate the efficacy of tucidinostat-based therapy in hormone receptor-positive metastatic breast cancer patients previously treated with CDK4/6 inhibitor therapy in the real world.

Cyclin-dependent kinase (CDK) 4/6 inhibitors have changed the treatment pattern of patients with hormone receptor-positive HER2-negative advanced breast cancer significantly prolongling patients'PFS and OS,but there is no optimal therapy after its progression.Tucidinostat (formerly known as chidamide) is an oral subtype-selective histone deacetylase inhibitor, ACE trial demonstrated that tucidinostat plus exemestane improved PFS compared with placebo plus exemestane in HR+,MBC patients that progressed after previous endocrine therapy,only 7 patients enrolled in the trial had previously received CDK4/6 inhibitors.The aim of this trial is to evaluate the efficacy of tucidinostat-based therapy in hormone receptor-positive metastatic breast cancer patients previously treated with CDK4/6 inhibitor therapy in the real world.

Inclusion Criteria:

• 1.woman, age > 18 years old 2.Diagnosed with HR+/HER2- Metastatic Breast Cancer 3.Patients received tucidinostat-based therapy after progression on CDK4/6 inhibitor 4.Complete medical history was available

Exclusion Criteria:

• 1.Medical history was incomplete

Patients diagnosed with breast cancer (according to International Classification of Diseases-10, ICD-10) with confirmed metastasis, regardless of being de novo diagnosed or progressed from a non-metastatic stage.