Breast Cancer HER2 Status

About 15% to 20% of breast tumors have higher levels of a protein known as HER2. These cancers are called HER2-positive breast cancers. Ask your cancer care team about your HER2 status and what it means for you.

What is HER2 and what does it mean?

HER2 is a protein that helps breast cancer cells grow quickly. Breast cancer cells with higher than normal levels of HER2 are called HER2-positive. These cancers tend to grow and spread faster than breast cancers that are HER2-negative, but are much more likely to respond to treatment with drugs that target the HER2 protein.

All invasive breast cancers should be tested for HER2 either on the biopsy sample or when the tumor is removed with surgery.

How are breast tumors tested for HER2?

Either a test called an immunohistochemistry (IHC) test or fluorescence in situ hybridization (FISH) test is used to find out if cancer cells have a high level of the HER2 protein. 

See Testing Biopsy and Cytology Specimens for Cancer and Understanding Your Pathology Report: Breast Cancerto get more details about these tests.  

What do the test results mean?

The results of HER2 testing will guide you and your cancer care team in making the best treatment decisions.

It is not clear if one test is more accurate than the other, but FISH is more expensive and takes longer to get the results. Often the IHC test is done first.

• If the IHC result is 0, the cancer is considered HER2-negative. These cancers do not respond to treatment with drugs that target HER2.
• If the IHC result is 1+, the cancer is considered HER2-negative. These cancers do not usually respond to treatment with drugs that target HER2, but new research shows that certain HER2 drugs might help in some cases (see below).
• If the IHC result is 2+, the HER2 status of the tumor is not clear and is called "equivocal." This means that the HER2 status needs to be tested with FISH to clarify the result.
• If the IHC result is 3+, the cancer is HER2-positive. These cancers are usually treated with drugs that target HER2.

Some breast cancers that have an IHC result of 1+ or an IHC result of 2+ along with a negative FISH test might be called HER2-low cancers. These breast cancers are still being studied but appear to benefit from certain HER2-targeted drugs.  

Triple-negative breast tumors don’t have too much HER2 and also don’t have estrogen or progesterone receptors. They are HER2-, ER-, and PR-negative. Hormone therapy and drugs that target HER2 are not helpful in treating these cancers. See Triple-negative Breast Cancer to learn more.

Triple-positive breast tumors are HER2-positive, ER-positive, and PR-positive. These cancers are treated with hormone drugs as well as drugs that target HER2.