HER2 is a protein on the surface of cells that, when present in unusually high amounts, drives cancer cell growth — and breast cancers that test “HER2-positive” can be effectively treated with targeted antibody therapies that block this protein.
Around 15–20% of breast cancers are HER2-positive1. The HER2 status is tested on the biopsy or operation specimen and reported as positive, negative, or “low” (a newer category). Targeted treatment with trastuzumab (Herceptin) and similar antibodies has transformed the outlook for HER2-positive cancer, which used to be considered one of the more aggressive subtypes.
Orientation Why you might be reading about this
You may have been told your cancer is HER2-positive (or negative), or you are reading about what receptor status means. HER2 is one of three key features pathologists report alongside ER and grade — and HER2-positive cancers in particular have specific, effective treatments tied to that status. This page explains what HER2 is, what testing involves, and what positive and negative results mean for treatment.
Related terms: Oestrogen receptor · Invasive ductal carcinoma · Core biopsy · Grade and stage
Definition What HER2 is
HER2 is a normal protein that sits on the surface of many cells in the body. It is part of a family of receptors that respond to growth signals — when activated, it tells the cell to divide. In normal cells, HER2 is present in modest amounts and is well-regulated.
In some breast cancers, the cancer cells produce far more HER2 protein than normal — sometimes 100 times more1. This over-expression is usually caused by extra copies of the HER2 gene (a process called gene amplification) and drives the cancer to grow faster than it otherwise would. Cancers with this feature are called HER2-positive4.
The clinical importance of HER2 is that it is a target for treatment. Antibody drugs that bind to HER2 — most prominently trastuzumab (Herceptin), alongside pertuzumab (Perjeta) and others — block the receptor and effectively switch off its growth signal. These targeted therapies have substantially improved outcomes for HER2-positive breast cancer over the past two decades.
Testing How HER2 is tested
HER2 status is tested on the biopsy or operation specimen as part of routine pathology. Two main testing methods are used:
Immunohistochemistry (IHC)
A staining test that shows how much HER2 protein is on the cell surface. Results are reported as a score from 0 to 3+4:
- 0 or 1+ — HER2-negative.
- 2+ — equivocal (uncertain). The result is confirmed with the second test (in situ hybridisation) before a final HER2 status is assigned.
- 3+ — HER2-positive.
In situ hybridisation (ISH or FISH)
A genetic test that counts how many copies of the HER2 gene are present4. Used to confirm 2+ IHC results, and increasingly used as the primary test in some labs.
A HER2-positive result requires either 3+ on IHC or amplification on ISH4.
“HER2-low” — a newer category
Recent advances in targeted therapy have introduced a third category: HER2-low (IHC 1+ or 2+ without amplification). HER2-low cancers were previously grouped with HER2-negative, but newer drugs (such as trastuzumab deruxtecan / Enhertu) have shown benefit specifically in this group5. Pathology reports increasingly include this distinction.
HER2-positive What HER2-positive means for treatment
For HER2-positive invasive breast cancer, targeted antibody therapy is part of the standard treatment plan:
- Adjuvant trastuzumab (after surgery), typically for 12 months136, is given to most HER2-positive patients with invasive disease.
- Pertuzumab is added to trastuzumab for higher-risk cases (larger tumours, lymph node-positive disease)37.
- Neoadjuvant therapy (chemotherapy plus trastuzumab plus pertuzumab, given before surgery) is increasingly used for HER2-positive cancers larger than 2 cm or with positive lymph nodes3.
- Newer agents — trastuzumab emtansine (Kadcyla)8, trastuzumab deruxtecan (Enhertu)5 — are used in higher-risk situations or for residual disease after neoadjuvant treatment8.
The combination of chemotherapy and HER2-targeted antibody is a more involved treatment than for HER2-negative cancer, but it is also markedly more effective for the HER2-positive subgroup. Survival outcomes for HER2-positive breast cancer have improved substantially since trastuzumab entered routine use6.
HER2-negative What HER2-negative means
For HER2-negative invasive breast cancer, treatment depends on the other receptor status (ER) and on size, grade, and lymph node involvement. HER2-negative cancers do not benefit from trastuzumab (it does not work without the HER2 target). Treatment is built around hormone therapy (for ER-positive cancers) and chemotherapy where the absolute benefit is meaningful.
A subset of HER2-negative cancers are also ER-negative and PR-negative — known as triple-negative breast cancer (TNBC). TNBC is treated primarily with chemotherapy and, in selected cases, immunotherapy and PARP inhibitors.
At consultation What to discuss at consultation
Once HER2 status is known:
- What the result means for the proposed treatment plan — particularly whether neoadjuvant chemotherapy plus targeted antibodies is being recommended.
- The HER2-low category if your IHC was 1+ or 2+ without amplification — this can affect treatment options if the disease ever recurs.
- The schedule and side effects of trastuzumab, where applicable.
- Cardiac monitoring during trastuzumab treatment — trastuzumab can affect heart function9 and is monitored with regular echocardiograms10.
If you already have a treatment plan and would like an independent review of what HER2 status means for it, you can book a second-opinion consultation.
Resources Further reading
- NHS — Breast cancer in women: Treatment — UK patient overview covering targeted therapy.
- Breast Cancer Now — HER2 — patient-focused guide.
- Macmillan Cancer Support — Trastuzumab (Herceptin) — patient guide to the targeted therapy.
- Breastory: Breast cancer surgery · Glossary: oestrogen receptor · Glossary: invasive ductal carcinoma