Illustration of a monoclonal antibody (red) inhibiting human epidermal growth factor receptor 3 (HER3, blue) on a cell membrane (purple and orange). HER3 belongs to the epidermal growth factor receptor (EGFR) family of proteins. It is normally crucial to cellular proliferation, survival, and differentiation, especially during development of certain tissues. When it is expressed abnormally, however, it can increase cancer cell growth, proliferation and resistance to chemotherapy. As such, HER3 is a target for certain cancer treatments.