Understanding HER2
HER2 (Human Epidermal Growth Factor Receptor 2) is a protein that promotes cell growth. Normally, cells have two copies of the HER2 gene and a moderate amount of the HER2 protein on their surface. In HER2-positive breast cancer, the HER2 gene is amplified — producing too much HER2 protein, which causes cancer cells to grow and divide more rapidly.
About 15-20% of breast cancers are HER2-positive.
How Is HER2 Tested?
HER2 status is determined by testing tumor tissue (from a biopsy or surgical specimen) using two methods:
- IHC (Immunohistochemistry): Measures the amount of HER2 protein. Scored 0, 1+, 2+, or 3+. A score of 3+ is HER2-positive.
- FISH (Fluorescence In Situ Hybridization): Measures the number of HER2 gene copies. Used to clarify IHC 2+ results.
HER2-Positive vs HER2-Low
A newer category — HER2-low — includes tumors with IHC 1+ or IHC 2+/FISH-negative. These tumors were previously classified as HER2-negative but may benefit from newer HER2-targeted antibody-drug conjugates like trastuzumab deruxtecan (Enhertu).
Targeted Treatments for HER2-Positive Cancer
- Trastuzumab (Herceptin): The first HER2-targeted therapy, a monoclonal antibody
- Pertuzumab (Perjeta): Often combined with trastuzumab
- T-DM1 (Kadcyla): An antibody-drug conjugate
- Trastuzumab deruxtecan (Enhertu): A newer, highly effective ADC
- Lapatinib (Tykerb) and tucatinib (Tukysa): Oral targeted therapies
Prognosis
HER2-positive breast cancer was historically considered aggressive, but the development of HER2-targeted therapies has dramatically improved outcomes. Many patients with HER2-positive breast cancer — even at advanced stages — achieve excellent responses to treatment.