HR-positive/HER2-negative early high-risk and advanced/metastatic breast cancer is one of the principal clinical settings represented for Abemaciclib. Inhibits CDK4 and CDK6, reducing RB phosphorylation and G1-to-S cell-cycle progression. The pharmacology is centered on cell-cycle control, with inhibition of CDK4/6 limiting progression through the G1-to-S transition. The medicine is classified as Targeted Therapy; CDK4/6 Inhibitor and belongs to the Selective CDK4/6 inhibitor family. In practice, its use extends to HR-positive/HER2-negative early high-risk and advanced/metastatic breast cancer; selected ESR1-mutated metastatic disease in combination with endocrine therapy where applicable. The tumor types represented here include Breast Cancer. Molecular and disease-specific eligibility can therefore be more important than tumor location alone when the treatment is considered. Treatment may be organized around Endocrine therapy + abemaciclib; adjuvant abemaciclib + endocrine therapy; metastatic HR+/HER2- endocrine combinations. These combinations show how the agent is incorporated into disease-specific treatment rather than used as a stand-alone concept across all cancers. The indication should be interpreted together with disease stage, histology, previous treatment, and any molecular or receptor requirement stated in the record. The mechanism also helps distinguish this medicine from other agents used for similar tumors. Current clinical use can vary by jurisdiction and by changes in regulatory labeling or guideline recommendations. This entry is intended as a reference summary, not as individualized prescribing advice.