HER2 status test

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HER2 is the more common name for the gene called ERBB2. HER2 stands for human epidermal growth factor receptor 2.

HER2 controls a protein on the surface of cells that helps them grow. If the number of HER2 gene copies increases, it may change how cells behave.

Normal cells have 2 copies of the HER2 gene. Some types of cancer have too many copies of the HER2 gene in a cell, this leads to too much HER2 protein being made. Having too many copies of the gene is called amplification, and having too much HER2 protein being made is called overexpression of HER2. When the protein is overexpressed, tumour cells can grow and multiply too quickly.

The most common cancers that are tested for HER2 in Canada are breast and stomach (gastric) cancers. Research shows that other types of cancer, such as ovarian, uterine, bladder and lung, may also overexpress HER2. More research is needed to understand how HER2 overexpression affects these tumours.

Why the HER2 status test is done

HER2 status testing is done to find out how much HER2 a tumour makes. Knowing this helps your healthcare team plan your treatment.

HER2 status testing for breast cancer is done on the main, or primary, breast tumour at the time of diagnosis, along with the hormone receptor status test. Because HER2 status sometimes changes over time or with treatment, testing may be done again if the breast cancer comes back (recurs) or spreads to another part of the body (metastasizes).

HER2 status testing for advanced stomach cancer is also done on the primary tumour. It is done for advanced cancers that are in the area where the esophagus joins the stomach (called the gastroesophageal or GE junction).

Testing for HER2 in cancers other than breast and the GE junction is not done very often in Canada except as part of a clinical trial.

How the HER2 status test is done

HER2 status testing is done on a biopsy sample taken from the tumour. There are 2 tests used to determine HER2 status.

Immunohistochemistry (IHC) measures the amount of HER2 protein in the cancer cells. Special antibodies attach to the HER2 protein and are linked to a coloured marker that can been seen with a microscope.

Fluorescence in situ hybridization (FISH) is a molecular genetic test that looks at the number of copies of the HER2 gene in the cancer cells. It uses a fluorescent probe that glows and attaches to certain parts of the chromosomes.

If the test results aren't clear with one test, the other test may be used to get a more definite result.

What the results mean

With IHC testing, the results are reported on a scale from 0 to 3+.



0 or 1+

The HER2 level has not increased.

The tumour is HER2 negative (0) or HER2-low (1+).


The amount of overexpression of HER2 is unclear.

A FISH test will be done to confirm the HER2 status.


The HER2 level is higher than normal.

The tumour is HER2 positive.

FISH test results are reported as negative or positive.

FISH negative means that the levels of the HER2 gene in the cells are normal, and the tumour is HER2 negative.

FISH positive means that on average there are at least 4 copies of the HER2 gene in the cells, and the tumour is HER2 positive.

HER2-positive cancers are more aggressive, which means that they tend to grow and spread more quickly than HER2-negative cancers. HER2-positive cancers usually have a higher grade.

What happens if HER2 overexpression is found

Your healthcare team will use the results of the HER2 status test to help plan your treatment. The treatment offered will depend on the type of cancer, the level of HER2 overexpression in the tumour and other factors, such as its stage and your overall health and preferences.

Expert review and references

  • Penelope Barnes, MD, FRCPC
  • American Cancer Society. Breast Cancer HER2 Status. 2022: www.cancer.org.
  • Idossa D, Borrero M. ERBB2-low (also known as HER2-low) breast cancer. JAMA Oncology. 2023: 9(4): 576.
  • US National Library of Medicine. Medline Plus Medical Tests: HER2 (Breast Cancer) Testing. Bethesda, MD: US Department of Health and Human Service; 2021: https://medlineplus.gov/encyclopedia.html.
  • Modi S, Jacot W, Yamashita T, Sohn J, Vida M, et al . Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer. New England Journal of Medicine. 2022: 387: 387-320.
  • Wolff aC, Somerfield MR, Dowsett M, Hammond EH, Hayes, DF, et al. Human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology - College of American Pathologists guideline update. Archives of Pathology & Laboratory Medicine. 2023.

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