HER2-positive breast cancer is a type of breast cancer characterized by the overexpression of the HER2 protein, which promotes the growth of cancer cells. This subtype accounts for about 15-20% of all breast cancer cases and tends to be more aggressive than other types. Diagnosis typically involves testing the tumor for HER2 levels through methods like immunohistochemistry or fluorescence in situ hybridization (FISH). Treatment options often include targeted therapies, such as trastuzumab (Herceptin), which specifically attack HER2 proteins, along with traditional methods like chemotherapy and radiation.