Abemaciclib is a medication used primarily to treat certain types of breast cancer, particularly those that are hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+/HER2-). It belongs to a class of drugs known as cyclin-dependent kinase (CDK) inhibitors, which work by blocking specific proteins that help cancer cells grow and divide. This drug is often prescribed for patients with advanced or metastatic breast cancer, either alone or in combination with other treatments like aromatase inhibitors. By inhibiting the CDK4 and CDK6 proteins, Abemaciclib helps slow down the progression of cancer, improving outcomes for many patients.