Human epidermal growth factor receptor-2 (HER-2/neu) is a genetic protein that is in part responsible for how certain cancer cells grow, divide, and repair themselves. Some women have cells with more than the normal two copies of the gene (HER-2) that produces this protein, which results in breast cancer that grows and spreads quickly and is likely to recur after treatment.

Tests can be performed on breast cancer cells to determine whether HER-2/neu protein is present. This information is useful when making treatment decisions.