HER2-positive means a tumor has unusually high levels of a protein called HER2 (human epidermal growth factor receptor 2) on the surface of its cells. This protein normally helps regulate cell growth, but when there’s too much of it, cells can grow and divide much faster than they should. About 15% to 20% of breast cancers are HER2-positive, and knowing your HER2 status is one of the most important factors in choosing the right treatment.
What HER2 Does in Normal and Cancer Cells
HER2 is a receptor protein that sits on the surface of many cell types throughout your body. In healthy tissue, it helps control how cells grow, divide, and repair themselves. The protein works by pairing up with other receptors in its family, which triggers internal signals telling the cell to grow or survive.
What makes HER2 unusual is that it doesn’t need an outside signal to become active. It can stay in an “open” position, ready to pair up with other receptors at any time. When a cancer cell has too many copies of HER2, those growth signals become constant and overwhelming. The cell essentially gets stuck with the accelerator pressed to the floor. Two key signaling chains drive this: one promotes rapid cell multiplication, and the other blocks the cell’s built-in self-destruct mechanism, letting damaged cells survive when they normally wouldn’t.
Gene Copies vs. Protein Levels
HER2-positive status can come from two related but distinct problems. Most often, the cancer cell has extra copies of the HER2 gene itself, a situation called gene amplification. More gene copies means the cell produces far more HER2 protein than normal. About 90% to 95% of breast cancers that overexpress HER2 do so because of this gene amplification. In rare cases (around 3%), cells overexpress HER2 protein without having extra gene copies, usually due to other molecular changes that crank up production.
This distinction matters because doctors use two different types of tests to check HER2 status, and understanding which problem is present helps ensure accurate results.
How HER2 Status Is Tested
Testing starts with a tissue sample from a biopsy or surgery. The first test is typically immunohistochemistry (IHC), which stains the tissue to measure how much HER2 protein is on the cell surfaces. Results are scored on a scale from 0 to 3+:
- Score 0: No staining, or barely detectable staining in 10% or fewer of tumor cells. This is negative.
- Score 1+: Faint, incomplete staining in more than 10% of tumor cells. This is also negative.
- Score 2+: Weak to moderate staining in more than 10% of cells, or strong staining in 10% or fewer. This is equivocal, meaning it’s unclear.
- Score 3+: Strong, complete staining around the full membrane of more than 10% of tumor cells. This is positive.
When IHC comes back as 2+ (equivocal), a second test called in situ hybridization (ISH or FISH) is used to count the actual number of HER2 gene copies in the cells. A ratio of 2.0 or higher confirms gene amplification. So a tumor is officially HER2-positive if it scores 3+ on IHC, or 2+ on IHC with a positive FISH result.
HER2-Low: A Newer Category
Until recently, HER2 status was a simple yes or no. That changed after a landmark clinical trial called DESTINY-Breast04, published in 2022, showed that a newer drug could also help patients whose tumors had low but not zero levels of HER2. This created a new category called HER2-low, defined as IHC 1+ or IHC 2+ with a negative FISH result.
This was a significant shift. Distinguishing between a score of 0 and a score of 1+ had never mattered clinically before. Now it does, because patients with HER2-low tumors may qualify for a targeted drug that was previously reserved only for HER2-positive cancers. The 2023 updates from ASCO and CAP (the organizations that set testing standards) reflect this change, moving away from the old binary system. Pathologists now need to be more precise about whether a tumor truly has zero HER2 expression or just a small amount.
How HER2-Positive Cancer Is Treated
A HER2-positive result is actually good news in one important sense: it gives your treatment team a clear molecular target. HER2-targeted therapies have dramatically improved outcomes for these cancers over the past two decades.
The backbone of treatment involves monoclonal antibodies, which are engineered proteins designed to latch onto the HER2 receptor and block its growth signals. Trastuzumab was the first of these and remains a cornerstone of care. Pertuzumab is often used alongside it, binding to a different spot on the receptor to prevent it from pairing with other receptors. These are typically given in combination with chemotherapy.
A newer class of drugs called antibody-drug conjugates (ADCs) takes this approach further. These medications combine a HER2-targeting antibody with a powerful cell-killing chemical. The antibody acts like a guided missile: it finds and attaches to HER2 on the cancer cell, gets pulled inside the cell, and then releases its toxic payload internally. This concentrates the drug’s effect where it’s needed and reduces some of the widespread damage that traditional chemotherapy causes. Two ADCs are currently approved for HER2-positive breast cancer, and one of them also works in HER2-low disease.
Heart Monitoring During Treatment
One important side effect of HER2-targeted therapies is that they can affect heart function. HER2 receptors are also present on heart muscle cells, so blocking them can sometimes weaken the heart’s pumping ability. This is different from the heart damage caused by certain chemotherapy drugs, because it’s often reversible when caught early.
Current guidelines recommend checking heart function with an echocardiogram (an ultrasound of the heart) every 3 months during the first year of treatment. If your pumping capacity drops by more than 10 percentage points to below 50% (the normal range is 55% to 70%), treatment is typically paused until heart function recovers. After the first year, monitoring frequency depends on your individual risk factors, generally every 6 months for the second year and annually after that. Patients with pre-existing heart conditions or other cardiovascular risk factors may need closer surveillance throughout.
HER2 in Cancers Beyond the Breast
HER2 overexpression isn’t exclusive to breast cancer. It also plays a role in gastric (stomach) and esophageal cancers, where trastuzumab is used as part of treatment. Testing in gastric cancer follows the same IHC and FISH framework, but the scoring rules differ in a couple of key ways. Gastric cancer cells can score as positive even with incomplete membrane staining, which would not qualify in breast cancer. Gastric tumors also tend to be more heterogeneous, meaning HER2 levels can vary significantly from one area of the tumor to another. Up to one-third of gastric cancers show this patchwork pattern, which can make testing trickier and sometimes requires sampling multiple areas.
What HER2-Positive Means for Prognosis
Before targeted therapies existed, HER2-positive breast cancer carried a worse prognosis than HER2-negative disease. A study from MD Anderson Cancer Center looking at small, node-negative tumors (the most favorable scenario) found that 5-year recurrence-free survival was 77% for HER2-positive patients compared to 94% for HER2-negative patients. The rate of cancer spreading to distant sites was 86% versus 97% over the same period.
Those numbers predate widespread use of modern HER2-targeted therapies, which have substantially closed that gap. Today, with appropriate treatment, many HER2-positive cancers respond well and have outcomes comparable to or better than some other breast cancer subtypes. The aggressive biology that once made these cancers dangerous is now what makes them vulnerable to treatment: the more a tumor depends on HER2 signaling, the more effectively that signal can be shut down.