Do the cancer cells make extra HER2?

Some breast cancer cells make too much of a protein called human epithelial growth factor-receptor 2, also called HER2. Healthy breast cells make some HER2, but the cancer cells can make a lot more. Tests in the lab can show whether your breast cancer cells make extra HER2.

Tests for HER2 can show whether your breast cancer is:

• HER2 positive. Breast cancer cells that make a lot of extra HER2 protein are called HER2 positive. Targeted therapy treatments with medicine that hurts cells making too much HER2 can help control this kind of breast cancer.
• HER2 low. Breast cancer is called HER2 low if some of the cancer cells show signs of making extra HER2. This result can happen if only some of the cells are making extra HER2. It also might mean the cells making HER2 are making low levels of the protein. Targeted therapy against HER2 may be a treatment option for HER2 low breast cancers in certain situations.
• HER2 negative. If the breast cancer cells don't make extra HER2, then the cancer is HER2 negative. Targeted therapy against the HER2 protein won't work for these breast cancers.

About half of breast cancers are believed to be HER2 low. Treatment for HER2-low breast cancer is an active area of cancer research.