HER2

HER2 (Human Epidermal growth factor Receptor 2) is a protein that sits on the surface of cells and acts as a receiver for signals telling the cell to grow and divide. In normal tissue, HER2 plays a controlled role in cell development. In certain breast cancers, the gene encoding HER2 (also called ERBB2) is amplified — meaning many extra copies are present — and the resulting overproduction of HER2 protein sends a constant, unchecked signal for the cancer cells to proliferate. This makes HER2-positive tumors tend to grow faster and be more likely to spread than HER2-negative tumors.

HER2 status is determined by examining a sample of tumor tissue using one or both of two methods: immunohistochemistry (IHC), which measures the amount of HER2 protein on the cell surface, and fluorescence in situ hybridization (FISH), which counts the number of copies of the HER2 gene. A tumor is classified as HER2-positive if it shows protein overexpression (3+ on IHC) or gene amplification confirmed by FISH. Approximately 15 to 20 percent of breast cancers fall into this category.

The identification of HER2 as a driver of cancer growth led directly to the development of targeted therapies designed to block it. Trastuzumab (Herceptin) was the first such drug — a monoclonal antibody that attaches to HER2 receptors and impairs their signaling. Since its approval in the late 1990s, multiple additional HER2-targeted agents have been developed, including pertuzumab, T-DM1, lapatinib, and neratinib. These therapies have dramatically changed the prognosis for HER2-positive breast cancer, turning what was once one of the most feared subtypes into one with relatively good outcomes when diagnosed and treated appropriately.

HER2 status is one of the most important biological characteristics of a breast cancer diagnosis — and one of the clearest examples of how precision medicine can change outcomes. When HER2-positive breast cancer was first characterized, it was associated with aggressive behavior and poor prognosis. The development of targeted anti-HER2 therapies changed that story profoundly. Today, women with early-stage HER2-positive breast cancer who receive anti-HER2 therapy alongside standard treatment have outcomes that compare favorably with other subtypes.

Understanding your tumor's HER2 status means understanding which treatments are specifically relevant to your cancer. If you are HER2-positive, anti-HER2 therapy is typically part of the treatment plan from the outset — not an add-on to consider later. Asking your oncologist about your HER2 result, what it means for your treatment, and what the expected benefits of HER2-targeted therapy are in your specific situation puts you in a much stronger position to engage with your care actively and confidently.