Breast Cancer

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Breast Cancer HER2 Status

HER2-positive breast tumors have higher levels of HER2 protein. About 15% to 20% of breast cancers are HER2-positive. Knowing if your breast cancer is HER2-positive can help guide your treatment. Ask your cancer care team about your HER2 status and what it means for you.

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What is HER2?

HER2 is a protein that helps breast cancer cells grow quickly. Breast cancers with higher 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. 

Often the IHC test for HER2 is done first. IHC for HER2 uses lab-made antibodies (immune system proteins) which only bind to HER2, and a chemical reaction causes a color change (stain) that can be seen under a microscope.

Results of IHC testing might be:

  • 0, no staining was seen on the test or staining was seen on less than 10% of cells
  • 1+, faint or incomplete staining was seen on more than 10% of cells
  • 2+, more than 10% of cells had darker or more complete staining than 1+ but not as strong as 3+
  • 3+, means more than 10% of cells stained completely and intensely on the test

FISH testing might be done after IHC to further classify the tumor type.

Results of the FISH testing might be:

  • Positive, there are more copies of the HER2 gene per cell than normal
  • Negative, there are no more copies of the HER2 gene per cell than normal

Biopsy and Cytology Tests

Learn more about the tests that might be done on your breast cancer biopsy.
Your Breast Pathology Report: Breast Cancer

Learn more about some of the medical terms you might see in your breast cancer pathology report.

What do HER2 test results mean?

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

If the IHC result is 0 with no membrane staining, the cancer is considered HER2-negative. These cancers do not respond to treatment with drugs that target HER2.

If the IHC result is 0 with membrane staining (HER2+ staining in more than 0% but no more than 10% of tumor cells), the cancer is considered HER2-ultralow. Some HER2-ultralow cancers might respond to drugs called antibody-drug conjugates.

If the IHC is 1+ or if the IHC is 2+ but FISH is negative, the cancer is considered HER2-low. Some HER2-low cancers might respond to drugs called antibody-drug conjugates.

If the IHC is 2+ and the FISH is positive or the IHC is 3+, the cancer is HER2-positive. These cancers are usually treated with drugs that target HER2.

Triple-negative breast cancers don’t have too much HER2 and don’t have estrogen or progesterone receptors. 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 cancers are HER2-positive, ER-positive, and PR-positive. These cancers are treated with hormone therapy as well as drugs that target HER2.

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Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).

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Last Revised: July 23, 2026

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