Definition

A type of breast cancer in which the cancer cells have a small amount of a protein called HER2 on their surface and do not have more than two copies of the HER2 gene. HER2-low breast cancer has usually been treated like HER2-negative breast cancer because the cancer cells do not make enough HER2 protein for drugs that target HER2 to work. However, newer drugs that target HER2 have been shown to be effective in treating HER2-low breast cancer.