What is HER2? Understanding a key biomarker in cancer

If you’ve recently been diagnosed with breast cancer, you may have heard your doctor mention something called “HER2.” It can sound technical and overwhelming at first, but understanding HER2 is an important step in making sense of your diagnosis and treatment options. HER2 stands for human epidermal growth factor receptor 2. It’s a protein found on the surface of some cells in the body. In healthy cells, HER2 plays a normal role in helping cells grow and repair themselves. But in certain cancers, especially breast cancer, HER2 can become overactive. When this happens, cancer cells make too much of the HER2 protein, and this results in what we call HER2-positive cancer. The extra HER2 signals tell cancer cells to grow and divide more quickly than they should. As a result, HER2-positive cancers tend to be more aggressive. They may grow faster and are more likely to spread if not treated effectively.

Not all breast cancers are HER2-positive, in fact only  about 10 to 20 percent of tumors have high levels of HER2. The rest are considered HER2-negative. This distinction matters because it directly affects which treatments are likely to work. Testing for HER2 is a standard part of diagnosing breast cancer. It is usually done on a sample of the tumor tissue. The most common tests are called IHC (immunohistochemistry) and ISH (in situ hybridization). These tests measure how much HER2 protein is present or whether the HER2 gene is amplified. Results are typically reported as 0, 1+, 2+, or 3+. A score of 3+ is considered HER2-positive. A score of 0 or 1+ is HER2-negative. A 2+ result is considered borderline and often requires additional testing to confirm. In recent years, you may also hear the term “HER2-low.” This refers to cancers that are not fully HER2-positive but still have some level of HER2 expression, usually classified as 1+ or 2+ without gene amplification. This category is becoming more important as new treatments are developed.

It is also important to know that HER2 status can sometimes change over time, especially if cancer comes back or spreads. That is why repeat testing may be recommended in certain situations. The reason knowing the status of your tumor’s HER2 level matters so much is because it can be targeted with specific therapies. These treatments are designed to block the HER2 signal and slow or stop cancer growth. Examples include drugs like trastuzumab (Herceptin) and trastuzumab deruxtecan (Enhertu). These therapies have changed the outlook for many patients with HER2-positive cancer. In the past, HER2-positive breast cancer was considered one of the more difficult types to treat. Today, targeted therapies have significantly improved outcomes and survival.

If you are unsure about your HER2 status, it is worth asking your care team. Knowing this information can help you better understand your options and feel more confident in your care.

At Outcomes4Me, we believe that clear, evidence-based information helps patients take an active role in their treatment decisions. Our platform can help you track your biomarker results, learn what they mean, and explore personalized treatment options based on the latest clinical guidelines.