HER2 in Breast Cancer

by Jason Wasserman MD PhD FRCPC
March 19, 2026

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

To determine eligibility for HER2-targeted therapy. Several drugs specifically designed to block the HER2 protein — including trastuzumab (Herceptin), pertuzumab (Perjeta), trastuzumab emtansine (T-DM1 / Kadcyla), trastuzumab deruxtecan (Enhertu), lapatinib (Tykerb), tucatinib (Tukysa), and neratinib (Nerlynx) — are effective specifically in HER2-positive cancers. Without knowing HER2 status, these treatments cannot be appropriately selected.
To classify the cancer. HER2 status, combined with estrogen receptor (ER) and progesterone receptor (PR) results, places the cancer into a molecular subtype that shapes the entire treatment plan.
To assess prognosis. HER2-positive cancers tend to grow faster and spread more readily than HER2-negative cancers. However, with modern HER2-targeted treatments, outcomes for HER2-positive breast cancer have improved dramatically — what was once considered one of the more aggressive subtypes is now among the most treatable.
To guide treatment in metastatic disease. In breast cancer that has spread to other parts of the body, HER2 status directs the sequence of targeted therapies and influences how long treatment is continued.

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

IHC 0. No staining, or faint, incomplete staining in less than 10% of tumour cells. The HER2 protein is essentially absent. This result is HER2-negative.
IHC 1+. Faint or barely perceptible incomplete staining in more than 10% of tumour cells. A small amount of HER2 protein is present on part of the cell surface. This result is HER2-negative by traditional criteria but is now classified as HER2-low — an important distinction for newer antibody-drug conjugate therapies.
IHC 2+. Weak to moderate complete staining of the entire cell membrane in more than 10% of tumour cells, or strong complete staining in 10% or fewer of tumour cells. This result is equivocal — it is neither clearly positive nor clearly negative — and requires FISH testing to resolve.
IHC 3+. Strong, complete, intense staining of the entire cell membrane in more than 10% of tumour cells. This result is HER2-positive. FISH is generally not required unless the staining pattern is unusual.

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:

HER2/CEP17 ratio ≥ 2.0 with average HER2 signals ≥ 4.0 per cell: Gene amplified — HER2-positive.
HER2/CEP17 ratio ≥ 2.0 with average HER2 signals < 4.0 per cell: Requires correlation with IHC score to determine final status.
HER2/CEP17 ratio < 2.0 with average HER2 signals ≥ 6.0 per cell: Requires correlation with IHC score.
HER2/CEP17 ratio < 2.0 with average HER2 signals ≥ 4.0 and < 6.0 per cell: Requires correlation with IHC score.
HER2/CEP17 ratio < 2.0 with average HER2 signals < 4.0 per cell: Not amplified — HER2-negative (or HER2-low, depending on IHC).

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-positive (IHC 3+, or IHC 2+ with FISH-amplified). Approximately 15–20% of breast cancers are HER2-positive. The cancer overexpresses HER2 and is dependent on the HER2 pathway to grow. This result qualifies the patient for HER2-targeted therapy. In early-stage disease, trastuzumab and pertuzumab are given alongside chemotherapy to reduce the risk of recurrence substantially. In metastatic disease, a sequence of HER2-targeted drugs can achieve prolonged disease control. Despite being a biologically aggressive subtype, HER2-positive breast cancer has seen some of the most dramatic treatment advances of any cancer type in the past two decades.
HER2-low (IHC 1+, or IHC 2+ with FISH-negative). HER2-low cancers were historically classified as “HER2-negative.” Still, this category has been redefined with the approval of trastuzumab deruxtecan (Enhertu). This antibody-drug conjugate can reach and kill cells with even small amounts of HER2 on their surface. Clinical trials have shown meaningful benefit from trastuzumab deruxtecan in patients with HER2-low metastatic breast cancer that has progressed after other treatments. HER2-low cancers are not treated with traditional HER2-targeted drugs (trastuzumab, pertuzumab) in the same way as HER2-positive cancers, but they are no longer considered entirely HER2-irrelevant.
HER2-negative / HER2-zero (IHC 0). The cancer has no meaningful HER2 expression and is not expected to respond to HER2-targeted therapies, including antibody-drug conjugates currently approved for HER2-low disease. Treatment is guided by ER/PR status and other tumour features. Research is ongoing into whether even IHC 0 tumours might eventually benefit from next-generation HER2-targeted drugs.
Equivocal (IHC 2+, awaiting FISH). This result means the initial IHC test could not definitively classify the cancer as positive or negative. FISH testing will follow. An equivocal IHC result does not mean the cancer is HER2-positive — the majority of 2+ cases are non-amplified on FISH. Final HER2 status will be reported once FISH results are available, typically within a few days to one week.

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:

HER2-positive / hormone receptor-positive (ER+ or PR+, HER2+). Both hormone signalling and HER2 drive the cancer. Treatment includes both HER2-targeted therapy and endocrine (hormone-blocking) therapy, often with chemotherapy.
HER2-positive / hormone receptor-negative (ER−, PR−, HER2+). The cancer is driven primarily by HER2. Treatment centres on HER2-targeted drugs combined with chemotherapy. Endocrine therapy is not used.
Triple-negative (ER−, PR−, HER2−). The cancer does not express ER, PR, or HER2. Treated primarily with chemotherapy; immunotherapy is added in eligible patients. HER2-targeted drugs are not used unless the cancer is reclassified as HER2-low.
Luminal (ER+ and/or PR+, HER2−). The cancer is hormone receptor-positive and HER2-negative. Treated with endocrine therapy; chemotherapy is added based on grade, stage, and genomic testing results.

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

If HER2-positive: Your oncologist will discuss adding HER2-targeted therapy to your treatment plan. For early-stage disease, this typically means trastuzumab and pertuzumab given with chemotherapy before or after surgery (or both), followed by a year of trastuzumab. If residual cancer remains after surgery, the treatment may be changed to trastuzumab emtansine (T-DM1). For metastatic disease, the sequence of HER2-targeted drugs is tailored to your situation.
If HER2-low: Your oncologist will note this status. In early-stage disease, HER2-low does not currently change the treatment approach from standard HER2-negative. In metastatic disease that has progressed on other therapies, trastuzumab deruxtecan may be an option — ask your oncologist whether this is relevant to your situation.
If HER2-negative (IHC 0): Treatment will be guided by ER/PR status and other features. HER2-targeted drugs are not part of the plan at this time.
If equivocal (2+ awaiting FISH): No treatment decisions should be delayed while awaiting FISH results. Your team will communicate the final HER2 status once available.

Questions to ask your doctor

What is my HER2 IHC score, and if the result was 2+, what did the FISH test show?
Is my cancer HER2-positive, HER2-low, or HER2-negative?
If I am HER2-positive, which HER2-targeted drugs will be part of my treatment plan?
If I am HER2-low, are there any treatments that target HER2-low cancers that I should know about?
Will my HER2 status be retested if the cancer comes back or spreads?
What is my cancer’s overall molecular subtype, and what does that mean for my prognosis?
Are there clinical trials studying new HER2-targeted treatments that I might qualify for?