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 tissue for HER2 levels, which helps determine the most effective treatment options. Targeted therapies, such as trastuzumab (Herceptin), are often used to specifically attack HER2-positive cells, improving outcomes for patients with this diagnosis.