HER2 is a protein found on the surface of cells that helps regulate cell growth. Every person has the gene that makes HER2, and in normal amounts it plays a routine role in cell division and tissue repair. But in some cancers, the gene produces too many copies of itself or churns out excess protein, driving cells to grow and divide much faster than they should. About 15 to 20 percent of breast cancers are HER2-positive, and HER2 overexpression also shows up in stomach, esophageal, bladder, and other cancers.
How HER2 Works in Normal Cells
HER2 belongs to a family of receptor proteins that sit on the outer membrane of cells, waiting for growth signals. When those signals arrive, HER2 pairs up with a partner receptor (a process called dimerization) and triggers a chain reaction inside the cell that promotes growth and survival. Unlike its sibling receptors, which get pulled inside the cell and broken down after they fire, HER2 stays anchored at the surface and keeps signaling for extended periods. A network of stabilizing proteins holds it in place within specific zones of the cell membrane, making it an especially persistent growth promoter.
In a healthy cell, this signaling is tightly controlled. Problems arise when the HER2 gene (officially called ERBB2) gets amplified, meaning the cell carries far more copies of the gene than the usual two. More gene copies lead to more HER2 protein on the cell surface, which leads to stronger and longer growth signals. That’s the mechanism behind HER2-positive cancer: not a foreign invader, but a normal system stuck in overdrive.
How Doctors Test for HER2
HER2 status is determined from a tissue sample taken during a biopsy. Two main tests are used, and they measure different things. Immunohistochemistry (IHC) looks at how much HER2 protein is present on the surface of tumor cells. In situ hybridization (ISH, often called FISH) counts the actual number of HER2 gene copies inside the cell’s DNA. Together, these tests sort tumors into categories that guide treatment decisions.
The Scoring System
IHC results are reported on a scale from 0 to 3+:
- IHC 0: No detectable HER2 protein, or only trace staining in a very small number of cells.
- IHC 1+: Faint, incomplete protein staining on more than 10 percent of tumor cells.
- IHC 2+: Moderate staining that could go either way. These cases are sent for FISH testing to check whether the gene itself is amplified.
- IHC 3+: Strong, complete staining on more than 10 percent of cells. This is definitively HER2-positive.
A tumor is considered HER2-positive if it scores IHC 3+ or if it scores IHC 2+ and FISH confirms gene amplification. These are the tumors eligible for the full range of HER2-targeted treatments.
What HER2-Low Means
Until recently, any tumor that wasn’t HER2-positive was simply labeled HER2-negative. That grouping has become more nuanced. Tumors scoring IHC 1+ or IHC 2+ without gene amplification are now recognized as “HER2-low.” They don’t have enough HER2 to qualify as positive, but they do have some protein on the surface. This matters because newer drugs, particularly an antibody-drug conjugate called trastuzumab deruxtecan, have shown effectiveness in HER2-low tumors. Clinical trials demonstrated meaningful benefit for metastatic breast cancer patients with IHC 1+ or IHC 2+/FISH-negative results.
Researchers have even identified a category called “HER2-ultralow,” which includes tumors scored as IHC 0 that still show a whisper of protein staining below the 1+ threshold. Whether these patients benefit from HER2-targeted treatment is still being studied, but the key point is that HER2 status is no longer a simple positive-or-negative question. It’s more of a spectrum.
HER2-Positive Treatment Options
The discovery that HER2 was driving certain cancers led to one of the most successful stories in targeted therapy. Rather than relying solely on chemotherapy, doctors can now aim treatments directly at the HER2 protein. There are three main categories of HER2-targeted drugs.
Monoclonal Antibodies
These are lab-made proteins designed to latch onto the HER2 receptor and block its signaling. Trastuzumab, the first to be developed, binds to one part of the HER2 protein and is typically given alongside chemotherapy for one year after surgery. Pertuzumab targets a different part of HER2 and is often combined with trastuzumab before surgery to shrink tumors. A third antibody, margetuximab, is approved for patients whose cancer has progressed after earlier treatments.
Antibody-Drug Conjugates
These combine an antibody with a potent chemotherapy payload. The antibody acts like a homing device, delivering the drug directly to HER2-expressing cancer cells while sparing more of the healthy tissue. Two are widely used: one pairs trastuzumab with a chemotherapy agent and is used when cancer persists after initial treatment, while the newer trastuzumab deruxtecan has shown strong results in both HER2-positive and HER2-low metastatic disease.
Targeted Pills
Tyrosine kinase inhibitors are oral medications that work inside the cell, blocking the signals HER2 sends after it’s activated. Several are available. Tucatinib, approved in 2020, is notable for its ability to cross the blood-brain barrier, making it useful for patients whose cancer has spread to the brain. Neratinib is given after a year of trastuzumab to reduce the risk of recurrence. These drugs are generally used in combination with other therapies rather than alone.
Survival Rates by Stage
HER2-positive breast cancer was once associated with a worse prognosis, but targeted therapies have dramatically changed that picture. Current five-year relative survival rates, based on data from the National Cancer Institute’s SEER program (2016 to 2022), reflect how effective modern treatment has become.
For tumors that also express hormone receptors (HR+/HER2+), five-year survival is 99.5 percent for localized disease, 91.5 percent for regional spread (cancer in nearby lymph nodes), and 48.7 percent for distant metastatic disease. The overall five-year survival for this subtype is 92.2 percent.
For hormone receptor-negative, HER2-positive tumors (HR-/HER2+), the numbers are slightly lower: 97.8 percent for localized, 86.4 percent for regional, and 43.1 percent for distant disease, with an overall five-year survival of 87.0 percent. The gap between localized and distant disease underscores why early detection matters so much, though even metastatic outcomes continue to improve as newer treatments enter clinical use.
HER2 Beyond Breast Cancer
While breast cancer gets the most attention, HER2 overexpression occurs in other tumor types. In gastric and gastroesophageal junction cancers, roughly 20 percent of cases are HER2-positive, though reported rates vary widely depending on the population studied, ranging from 4 to 53 percent. Trastuzumab is approved for HER2-positive stomach cancer, and antibody-drug conjugates are being tested across additional tumor types including bladder, colorectal, and non-small cell lung cancers. Testing for HER2 is becoming standard in more cancer types as treatment options expand.
One complication in non-breast cancers is that HER2 protein expression can vary from one area of a tumor to another. This intratumor heterogeneity means a single biopsy sample might not capture the full picture, which is why some oncologists recommend testing multiple samples or using complementary testing methods when results are borderline.