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MyPathologyReport Printed: August 23, 2026

HER2 in Breast Cancer

HER2 (human epidermal growth factor receptor 2) is a protein found on the surface of cells that acts like an antenna, receiving signals that tell the cell to grow and divide. In normal breast tissue, HER2 is present in small amounts and plays a controlled role in cell growth. In some breast cancers, the gene that makes HER2 is amplified — meaning extra copies of the gene are present — causing the cancer cells to produce far too much HER2 protein. These cancers are called HER2-positive, and they tend to grow more aggressively than cancers without HER2 amplification. The critical finding for patients is that HER2-positive breast cancers can be treated with a group of highly effective drugs specifically designed to target the HER2 protein. Testing for HER2 is a standard part of every newly diagnosed invasive breast cancer workup, and the result has major implications for both treatment and prognosis.

What the test looks for

The HER2 gene (also known as ERBB2) is located on chromosome 17. In a normal cell, there are two copies of this gene — one on each chromosome. In HER2-positive breast cancers, the gene is amplified: the cancer cells carry many extra copies, sometimes dozens, which drive overproduction of the HER2 protein on the cell surface. The excess HER2 protein keeps growth signals constantly switched on, causing uncontrolled cell division.

HER2 testing determines whether a patient’s cancer has this amplification or overexpression. A positive result identifies a cancer that is dependent on the HER2 pathway for growth — and therefore potentially vulnerable to drugs that block it. A negative result means the cancer is not HER2-driven, and HER2-targeted drugs are unlikely to help.

There is also a category called HER2-low, which has emerged as clinically important in recent years. HER2-low cancers do not meet the traditional threshold for HER2-positivity but carry enough HER2 protein on the cell surface to be targeted by a newer class of drugs called antibody-drug conjugates. This distinction is now routinely reported by pathologists.

Why is the test done

How the test is performed

HER2 testing uses two complementary methods, and most patients will have both performed — either simultaneously or sequentially.

Immunohistochemistry (IHC)

Immunohistochemistry (IHC) is the first test usually performed. It uses antibodies to detect the HER2 protein directly on a thin slice of tumour tissue mounted on a glass slide. The antibodies attach to HER2 proteins on the cell surface, and a colour reaction makes them visible under the microscope. A pathologist then assesses the intensity and completeness of the staining pattern around the cell membrane and assigns a score of 0, 1+, 2+, or 3+.

Fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) is a molecular test that counts the number of HER2 gene copies in a cancer cell’s nucleus. Fluorescent probes bind to the HER2 gene and to chromosome 17, allowing a pathologist to count both and calculate a ratio. A high ratio (or a high absolute number of HER2 gene copies) indicates gene amplification. FISH is considered the gold standard for confirming HER2 amplification and is used when IHC results are equivocal (2+).

When each test is used

In most laboratories, IHC is performed first because it is faster and less expensive. If the IHC result is clearly positive (3+) or clearly negative (0 or 1+), no further testing is required. If IHC returns a score of 2+ (equivocal), FISH is automatically performed to determine whether gene amplification is present. Some laboratories perform FISH on all cases as a matter of protocol.

How results are reported

IHC scoring

FISH reporting

FISH results are reported as the ratio of HER2 gene signals to chromosome 17 (CEP17) signals, along with the average number of HER2 gene copies per cell. Current guidelines from the American Society of Clinical Oncology and the College of American Pathologists define five FISH groups, which are then interpreted in conjunction with the IHC score. In general:

Your pathology report will state the final combined interpretation — HER2-positive, HER2-negative, or HER2-low — after taking both the IHC and FISH results into account.

What each result means

HER2 status and breast cancer subtypes

HER2 testing is one of three pillars — alongside ER and PR — used to classify breast cancers into molecular subtypes:

Retesting HER2 in recurrent or metastatic disease

HER2 status can change between a primary tumour and a recurrence or metastasis. A cancer that was HER2-negative at diagnosis may become HER2-positive in a later biopsy, or vice versa. For this reason, guidelines recommend repeating HER2 testing — ideally from a new biopsy of a metastatic site — when the cancer spreads or returns. This ensures that treatment decisions reflect the biology of the cancer at its current stage.

What happens next

Questions to ask your doctor

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This article was printed from MyPathologyReport — https://www.mypathologyreport.ca/biomarkers/her2-in-breast-cancer/
The information in this article is intended to help patients understand their pathology report. Please discuss any questions with your healthcare provider.