HER-2/neu (also called c-erbB-2) is another special protein on breast cancer cells. Like the ER and PR it is a surface receptor protein, but unlike ER and PR it is not a "hormone" receptor but rather a "growth factor" receptor. When the HER-2 receptor binds with its partner (growth factor) from the bloodstream, this binding stimulates the growth of the tumor cells. HER-2 tends to be positive in high-grade tumors and tumors that are negative for ER and PR. HER-2 positivity, on the other hand, does predict response to the new "designer" chemotherapy drug called HERCEPTIN®. HERCEPTIN, which blocks the binding of HER-2 with its growth factor, does not work in tumors that are HER-2 negative.

All invasive breast cancers are tested for the presence or absence of this marker and the results are reported as 0 (negative, most favorable), 1+, 2+ or 3+ (positive, least favorable).