HER2-positive breast cancer is one of the principal clinical settings represented for Trastuzumab. Binds HER2, inhibits signaling and mediates immune effector mechanisms such as antibody-dependent cellular cytotoxicity. HER2 biology is central to treatment selection, and the clinical role depends on the tumor’s HER2 status and disease setting. The medicine is classified as Targeted Therapy; HER2-Directed Therapy; Monoclonal Antibody and belongs to the HER2 monoclonal antibody family. In practice, its use extends to HER2-positive breast cancer; HER2-positive gastric/GEJ cancer. The tumor types represented here include Breast; Gastric/GEJ. Molecular and disease-specific eligibility can therefore be more important than tumor location alone when the treatment is considered. Treatment may be organized around TCH; TCHP; THP; trastuzumab + taxane; trastuzumab + endocrine therapy; gastric/GEJ HER2 regimens. 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.