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HER2 Tumor Marker Test
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

HER2 Tumor Marker Test

HER2 tumor marker testing determines if cancer cells have high levels of the HER2 protein or gene, which helps guide targeted therapy decisions.

What is HER2 tumor marker testing?

HER2 (human epidermal growth factor receptor 2) is a protein involved in normal cell growth. Some cancers, including breast, stomach, and esophageal cancers, make larger-than-normal amounts of HER2. These cancers tend to grow quickly. HER2 targeted therapy can block these proteins, but this treatment only works for cancers that are HER2 positive.

The test measures either the amount of HER2 protein on the surface of cancer cells or the number of copies of the HER2 gene inside the cells.

Why do I need HER2 tumor marker testing?

Testing is used to guide treatment decisions for several types of cancer, including:

  • Breast cancer: Tested at initial diagnosis, if it spreads (metastatic), or if it recurs.
  • Stomach or esophageal cancer: Tested if the cancer cannot be removed by surgery or has returned after treatment.
  • Other cancers: Including pancreatic, bladder, and ovarian cancers, where HER2 levels may influence treatment options.

What happens during the test?

The test requires a sample of tumor tissue, which is obtained through a biopsy. Depending on the tumor's location, this may involve:

  • Needle biopsy: Using a hollow needle to remove cells or tissue.
  • Surgery: Removing part or all of the tumor.
  • Endoscopy: Using a scope to remove a tissue sample from the stomach or esophagus.

What do the results mean?

Results are reported based on the type of test performed:

  • IHC (Immunohistochemistry) test: Measures HER2 protein. A score of 0 or 1+ is HER2 negative. A score of 2+ is borderline (equivocal) and may require further testing. A score of 3+ is HER2 positive.
  • FISH (Fluorescence in situ hybridization) test: Checks for extra copies of the HER2 gene. Results are reported as either HER2 negative or HER2 positive.

If your cancer is HER2 positive, it is likely to respond to HER2 targeted therapy. If it is HER2 negative, your provider will discuss other treatment options.

Frequently Asked Questions

n Immunohistochemistry (IHC) score of 2+ is considered equivocal or borderline, meaning it is not definitively positive or negative. In this case, the lab must perform a secondary test called Fluorescence In Situ Hybridization (FISH) to count the number of HER2 gene copies. If the FISH test shows gene amplification, the cancer is classified as HER2 positive and eligible for targeted therapy. If FISH is negative, the cancer is considered HER2 negative. Does a HER2 negative result mean the cancer is less aggressive or easier to treat?::Not necessarily. HER2 status is a specific biological marker that determines eligibility for targeted therapies like trastuzumab, but it does not solely dictate the overall aggressiveness or prognosis of the cancer. A HER2 negative cancer can still be aggressive and require chemotherapy, radiation, or other treatments. The overall treatment plan depends on a combination of factors including tumor size, grade, lymph node involvement, and other biomarkers like estrogen and progesterone receptor status. Can HER2 status change over time, and does this require re-testing?::Yes, HER2 status can change, particularly in metastatic or recurrent breast cancer. A tumor that was HER2 negative at initial diagnosis may become HER2 positive upon recurrence, or vice versa. Because targeted therapies are only effective for HER2 positive tumors, clinicians often recommend re-biopsy and re-testing if the cancer recurs or spreads, especially if the original diagnosis was HER2 negative or if the patient is not responding to current treatment. How does HER2 testing differ for stomach cancer compared to breast cancer?::While the testing methods (IHC and FISH) are similar, the clinical context differs. In breast cancer, HER2 testing is standard for almost all invasive diagnoses to guide initial therapy. In stomach and esophageal cancers, HER2 testing is typically reserved for advanced, unresectable, or metastatic cases because HER2 positive status is less common (occurring in about 15-20% of cases). A positive result in gastric cancer opens the door to specific targeted combinations with chemotherapy that are not used in the early-stage treatment of breast cancer.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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