HER2-positive breast cancer is a highly malignant form of breast cancer, identified when the HER2 protein concentration exceeds the normal threshold on the surface of the cancer cell. One in five breast cancers will have cancer cells with mutations in genes that produce an excess of the HER2 protein. HER2-positive breast cancers tend to be more aggressive than other types of breast cancer. Patients are less susceptible to hormone therapy, although many people with HER2-positive breast cancer can still benefit from hormone therapy.