You're probably referring to a breast cancer that tests positive for a protein called human epidermal growth factor receptor-2 (HER-2), which promotes the growth of cancer cells. In about one of every three breast cancers, the cancer cells make an excess of this protein due to a gene mutation. This gene mutation can occur in many types of cancer — not only breast cancer. HER-2-positive breast cancers tend to be more aggressive and less responsive to hormone treatment. But a new drug therapy, trastuzumab (Herceptin), is available.

Breast tissue can be tested for HER-2. Routine testing of all breast cancers for HER-2 protein at initial diagnosis isn't recommended because the results don't affect treatment decisions. But if the cancer returns or spreads (metastasizes), a doctor will likely recommend testing for HER-2.

Treatment of HER-2 cancers may include Herceptin, which targets HER-2. This may slow the growth of the cancer and even decrease its size. Herceptin may be used as either a treatment by itself or combined with chemotherapy. Several ongoing clinical trials are examining the use of Herceptin in preventing cancer from ever returning (adjuvant therapy).