HER2 is one of four receptors in the EGFR family, which also includes HER1 (EGFR), HER3, and HER4. These receptors can form homodimers or heterodimers upon ligand binding, triggering various intracellular signaling cascades. HER2, however, is unique because it has no known ligand; it is activated through dimerization with other HER family members.
