What does it mean if I tested HER2-positive for breast cancer?

Aggressive. HER2 is a gene present in many cells that sends control signals causing the cells to grow, divide, and make repairs. A normal breast cell has 2 copies of the HER2 gene. Certain breast cancers start when a cell has more than 2 copies of that gene, and start over-producing the HER2 protein. The involved tumor cells grow and divide more quickly. These tumor cells are sensitive to Herceptin (trastuzumab).