HER2 Status and Breast Cancer

April 10, 2025

This page was reviewed under our medical and editorial policy by Leah Naghi, M.D., assistant clinical professor, and Susan Yost, Ph.D., staff scientist, Department of Medical Oncology & Therapeutics Research, City of Hope^® Cancer Center Duarte

When talking through a breast cancer diagnosis, a patient’s doctor will often share information about the cancer’s type and genetic makeup. This may include its human epidermal growth factor receptor 2 (HER2) status, a protein that is sometimes found in high amounts on breast cancer cells.

This guide to the HER2 protein and what it means for breast cancer treatment will help patients learn more.

What Is HER2?

HER2 is a type of protein that is found normally in certain cells of the body. In higher than normal amounts, it may encourage growth of cancer cells. HER2 stands for human epidermal growth factor receptor 2, and this protein plays a role in normal cell growth and division.

Doctors use HER2 as a tumor marker. HER2 is sometimes found in high amounts in breast cancer cells, so measuring HER2 levels in breast tumor tissue helps doctors learn more about a patient’s cancer.

In breast cancer, along with certain ovarian, stomach and esophageal cancers, high HER2 levels may cause the cancer to grow and spread quickly.

HER2 Testing

A patient’s doctor might recommend HER2 testing to learn more about their breast cancer type. HER2 testing may be performed for breast cancer to:

• Learn more about the genetic makeup of a newly diagnosed invasive breast cancer or a cancer that has metastasized and spread to another area of the body
• Learn more about the genetic makeup of a recurrent cancer, one that returns after the initial treatment has finished

If the breast cancer is not invasive, HER2 testing is not likely to be performed.

During HER2 testing, a tissue sample is taken from the tumor, either via a breast biopsy or during surgery to remove the breast cancer. Before the test, patients may need to fast for a few hours and also arrange for a friend or family member to help them get home. However, a patient’s care team will give detailed instructions on what to do before and after their HER2 test.

If the tissue sample is taken via a biopsy, the biopsy may be performed in several ways. These include the following.

Core needle biopsy: Using guided imaging, a large, hollow needle is used to remove a tissue sample.

Fine needle aspiration (FNA): A thin, hollow needle is used to remove a tissue or fluid sample. FNA has limitations in this setting, as a sample collected using this method can’t be analyzed for structure, which is generally important when making a diagnosis.

Surgery: A vacuum-assisted biopsy is a surgical procedure that may be used to take a tissue sample. It uses imaging, a gentle vacuum device and a small knife to gently take a tissue sample.

IHC Test

An immunohistochemistry (IHC) test is a laboratory technique doctors use to test HER2 levels in a patient’s tissue sample. During IHC testing, a pathologist uses a type of antibody that attaches to the HER2 protein in the tissue sample and causes a change in color.

The pathologist then examines the tissue sample under a microscope and looks for the telltale color change.

Based on the results, they may determine HER2 levels within the tumor. HER2 IHC results are scored as follows:

• 0 or 1+: HER2-negative
• 2+: Equivocal (additional testing may be required)
• 3+: HER2-positive

An IHC test is usually the first type of test used if doctors want to determine HER2 levels to help guide treatment decisions in breast cancer.

FISH Test

In addition to the IHC test, the laboratory technique known as fluorescence in situ hybridization (FISH) test may also be performed on the tissue specimen to determine the status of HER2 in breast cancer. During a FISH test for HER2, a pathologist uses a special type of DNA material that gives off a glow under a microscope. This DNA attaches to HER2 proteins and the sample is examined under a microscope to determine the number of HER2 gene copies present.

A FISH test that’s HER2-negative means there are fewer than two copies of the HER2 gene. HER2-equivocal means there are two to four copies of the HER2 gene and HER2-positive means there are more than four copies of the HER2 gene per chromosome.

FISH testing is known to be very accurate, but it is also more expensive than IHC testing. HER2-positive breast cancer is associated with an increased risk of cancer recurrence and metastasis. However, treatment with HER2-targeted therapies can significantly improve the prognosis for these patients.

HER2 Test Results

After a patient’s HER2 testing, it may take one to two weeks to get the results. However, the time it takes for results to be ready may vary based on the type of test performed. Generally, FISH test results take longer than other tests.

When the results are ready, the care team will explain what they mean. HER2 results are given as a score, which is a measure of HER2 levels.

HER2-Positive

If high levels of HER2 are found in the tumor, it is considered HER2 positive. The American Cancer Society estimates that about 15% to 20% of breast cancers are HER2-positive.

Knowing that a cancer is HER2-positive helps doctors determine which treatment options to recommend. While HER2-positive cancers are generally more aggressive than HER2-negative cancers, many treatment options are available.

Patients with HER2-positive breast cancers may be given targeted therapies with or without chemotherapy, specifically designed to target and destroy HER2 proteins. There are a range of targeted therapies, each working slightly differently but with the aim of inhibiting growth or

HER2-Negative

If testing finds low levels of HER2 (less than two copies of the HER2 gene), this is considered HER2-negative. This cancer type may be more slow-growing than HER2-positive cancers.

HER2-negative cancers have many treatment options, but are seldom treated with targeted HER2 therapy, as this cancer type does not respond to it. A patient’s doctor will recommend an evidence-based treatment plan, based on the type and stage of their breast cancer.

Most breast cancers are HER2-negative or HER2-low. According to researchers in a 2022 study in Breast, up to 20% of breast cancers are HER2-negative and up to 60% of breast cancers are classified as HER2-low.

HER2-Low

Testing may find that the breast cancer cells have a little bit of HER2 on the surface of the cells, but do not have more than two copies of the HER2 gene, indicating that the FISH test is negative. This is typically treated in the same way as HER2-negative breast cancers, but sometimes these cancers may respond to HER2 targeted therapy, such as fam-trastuzumab deruxtecan-nxki (Enhertu^®).