Illustration of human epidermal growth factor receptor (HER2, light blue) and HER3 (dark blue) forming a heterodimer (complex of two different molecules) on a human cell membrane (purple and orange). HER2 and HER3 belong to the epidermal growth factor receptor (EGFR) family of proteins. In normal cells they readily form this heterodimer which promotes cellular proliferation, survival, and differentiation, especially during development. When expressed abnormally, however, it can increase cancer cell growth, proliferation and resistance to chemotherapy. As such, HER2 and HER3 are therapeutic targets in the treatment of some cancers.