HER2 positive and HER2 negative breast cancer explained: testing, treatment, and what HER2 low means

A breast cancer pathology report contains a great deal of information, and HER2 status is one of the most important lines on it. This single detail helps guide which medicines are most likely to work, and it has become even more useful in the past few years because doctors now recognize more categories than a simple positive or negative label. Understanding what HER2 means, how it is measured, and how it shapes care can help anyone facing a breast cancer diagnosis feel more prepared for conversations with the care team.

What HER2 is and why it appears on the pathology report

HER2 stands for human epidermal growth factor receptor 2. It is a gene that every cell in the body carries, and its job is to produce a protein of the same name. That protein sits on the outer surface of a cell and works like a small antenna, receiving signals that tell the cell when to grow and divide. In a healthy cell, HER2 helps keep growth and repair in balance.

In some breast cancers, the cells make far too many copies of the HER2 gene. This causes them to display an unusually high number of HER2 antennas on the surface, and the cell receives a constant grow signal. HER2 is what specialists call a biomarker, which is a measurable feature of the cancer that helps guide care. A related read on what a biomarker is offers a helpful starting point for anyone new to the concept.

HER2 status is not passed down through families. It reflects a change inside the cancer cells themselves.

How doctors test for HER2

Testing is performed on tumor tissue already collected during a biopsy or surgery, so a separate blood draw is typically not required. According to the American Cancer Society, all invasive breast cancers should be tested for HER2. Two main tests are used.

The first is immunohistochemistry, often shortened to IHC. Lab-made antibodies are applied to a thin slice of tumor tissue and stain the HER2 protein so it can be seen under a microscope. A pathologist gives each sample a score of 0, 1+, 2+, or 3+ based on how strongly the cells are stained.

The second test is called in situ hybridization, or ISH, and the version that uses glowing DNA probes is known as FISH. This test counts how many copies of the HER2 gene each cancer cell has. It is generally used when the IHC score is 2+, which is considered borderline and needs an extra check.

Results are usually reported as HER2 positive or HER2 negative, but the picture is more layered today. Many pathology reports also flag HER2 low or HER2 ultralow, terms that describe cancers with small amounts of HER2 protein. Asking the care team for the exact IHC score, rather than only the positive or negative label, is worth doing. Symptoms that lead to a first breast cancer evaluation vary widely, and this overview of signs beyond a lump gives a broader view of what can prompt further testing.

HER2 positive breast cancer

A cancer is called HER2 positive when the pathology report shows an IHC score of 3+, or an IHC score of 2+ with a positive FISH result. The American Cancer Society estimates that about 15 to 20 percent of breast cancers fall into this group.

HER2 positive cancers tend to grow and spread faster than HER2 negative cancers. In the past, this reputation for being aggressive was a source of real concern. That picture has changed significantly. HER2 positive cancers are also much more likely to respond to medicines designed to lock onto the HER2 protein and block its growth signal, and outcomes have improved dramatically as those medicines have entered routine care.

Several classes of HER2 directed medicines are now used, including monoclonal antibodies that attach to HER2 on the outside of the cell, antibody drug conjugates that carry a chemotherapy payload directly to HER2 positive cells, and tyrosine kinase inhibitors that block the HER2 signal from inside the cell. These are prescribed and managed by the oncology team, and the mix depends on the stage of the disease, whether hormone receptors are also present, and how the cancer responds over time. Stage at diagnosis remains one of the most powerful drivers of outcome, and this explainer on breast cancer stages walks through how doctors assign a stage and what each stage generally means.

HER2 negative breast cancer, including HER2 low and HER2 ultralow

HER2 negative cancers make up the large majority of breast cancers, and this category is not one single disease. It includes hormone receptor positive cancers, which are fueled by estrogen and progesterone and are usually treated with hormone blocking therapy, sometimes paired with drugs called CDK4 and CDK6 inhibitors that slow cancer cell division. It also includes triple negative breast cancer, which lacks estrogen receptors, progesterone receptors, and HER2 overexpression, and which relies more on chemotherapy, immune checkpoint therapy, and newer antibody drug conjugates. This companion piece on triple negative breast cancer explains why this subtype behaves differently from other HER2 negative cancers.

The most significant recent shift in the HER2 negative category is the recognition of HER2 low and HER2 ultralow disease. HER2 low describes tumors with an IHC score of 1+, or an IHC score of 2+ with a negative FISH result. HER2 ultralow describes tumors with an IHC score of 0 that still show a faint stain in some cells. Peer reviewed reviews estimate that roughly half of all breast cancers now qualify as HER2 low, and a smaller subset qualify as HER2 ultralow.

Why does this matter? A newer antibody drug conjugate can treat some cancers in this range because it uses even a small amount of HER2 as a docking point to deliver its chemotherapy payload into the cell. In 2022, this option was expanded to HER2 low metastatic disease, and in early 2025 to certain HER2 ultralow hormone receptor positive metastatic cases. People once told their cancer was simply HER2 negative may now qualify for a HER2 directed option, which is a practical reason to ask the care team about the exact IHC score.

It is also worth knowing that HER2 status can change between the original tumor and a later recurrence. Retesting a new biopsy is sometimes recommended if the cancer returns or spreads to a new site.

Where clinical research fits in

Every HER2 directed medicine in use today was tested first in clinical research studies, and this includes the HER2 low and HER2 ultralow expansions. Research studies continue to explore earlier use of targeted drugs, new combinations, better ways to measure HER2 accurately, and fresh approaches for triple negative disease. Historically, only a small share of adults with cancer take part in a research study, which slows the pace of learning and limits how well findings apply across different communities. A related read on World Breast Cancer Research Day explores how research and participation continue to shape progress.

DecenTrialz is a clinical trial recruitment and pre-screening platform. People share information about themselves, are matched to studies through AI assisted matching, and are pre-screened by a registered nurse before being referred to a research site. The research site team owns final eligibility determination, study walk through, informed consent, and enrollment, not DecenTrialz. For anyone weighing whether a study might be worth exploring, having a copy of the full pathology report, including the exact IHC score and any FISH result, makes matching more accurate.

Frequently asked questions

Is HER2 positive worse than HER2 negative?

The comparison is more nuanced than it may seem. HER2 positive cancers tend to grow faster, yet they respond well to HER2 directed medicines, and outcomes have improved significantly over the past two decades. Some HER2 negative cancers, particularly triple negative disease, can be harder to treat. Stage at diagnosis, hormone receptor status, and how the cancer responds to care usually matter as much as HER2 status alone, and often more.

Can HER2 status change over time?

Yes. The HER2 result from the original tumor and a later recurrence do not always match, because tumors vary from area to area and can evolve. Some care teams recommend retesting a new biopsy if the cancer comes back or spreads.

What if the report says HER2 low or HER2 ultralow?

These are still classified as HER2 negative for most purposes, and traditional HER2 antibodies are not used. However, an antibody drug conjugate that targets even small amounts of HER2 may be an option in certain advanced settings. Asking the oncologist whether the exact score qualifies for any current options or research studies is reasonable.

Does HER2 testing require another procedure?

Usually it does not. Testing is performed on tumor tissue already collected during a biopsy or surgery, and results are added to the pathology report.

A clearer picture leads to a better conversation

Knowing whether a breast cancer is HER2 positive, HER2 negative, HER2 low, or HER2 ultralow no longer only labels the disease. It shapes the care plan, the medicines that may help, and, in some cases, the research studies that might be a fit. Requesting the full pathology report and asking the care team to walk through the exact score is a practical step anyone can take. That single conversation can open doors that a simple positive or negative line on a report might otherwise leave closed.