Previously, FDA have approved two cyclin-dependent kinases 4/6 (CDK4/6) inhibitors accompanied with endocrine therapy for HR (+)/HER2 (-) breast cancer treatment. However, the efficacy of these CDK4/6 inhibitors as monotherapy for advanced breast cancer treatment still await further investigation. Based on the results of MONARCH 1 trial, abemaciclib became the first CDK4/6 inhibitor approved by US FDA as a monotherapy for advanced HR(+)/HER2 (-) breast cancer patients, who were previously treated with hormone therapy and chemotherapy. The MORNARCH 1 trial is a phase II single arm study that recruited 132 advanced HR (+)/HER2 (-) breast cancer patients, who experienced treatment failure after hormone therapy and chemotherapy. The objective response rate (ORR) was 19.7% (95% CI, 13.3-27.5, n=26). The clinical benefit rate including complete response, partial response and stable disease over 6 months was 42.4%. In conclusion, abemaciclib will become a promising new treatment option for patients with advanced HR (+) /HER2 (-) breast cancer following multiple prior therapies.