HER2 (human epidermal growth factor receptor 2) is a protein that plays an important role in cell growth, multiplication, and survival. HER2 is produced by the HER2/neu gene. Normal cells contain two copies of the HER2 gene and make amounts of HER2 to allow normal cell growth and division. Some cancer cells, however, contain many more copies of the HER2 gene and in turn produce too much HER2 protein. This “overexpression” of HER2 receptors results in a cancer that is “HER2+.” About 20% to 25% of breast cancer tumors are HER2+ (HER2-positive).4
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