Pseudokinase domain of the Protein Tyrosine Kinase, HER3HER3 (ErbB3) is a member of the EGFR (HER, ErbB) subfamily of proteins, which are receptor PTKs (RTKs) containing an extracellular EGF-related ligand-binding region, a transmembrane helix, and a cytoplasmic region with a tyr kinase domain and a regulatory C-terminal tail. Unlike other PTKs, phosphorylation of the activation loop of EGFR proteins is not critical to their activation. Instead, they are activated by ligand-induced dimerization, leading to the phosphorylation of tyr residues in the C-terminal tail, which serve as binding sites for downstream signaling molecules. HER3 contains an impaired tyr kinase domain, which lacks crucial residues for catalytic activity against exogenous substrates but is still able to bind ATP and autophosphorylate. HER3 binds the neuregulin ligands, NRG1 and NRG2, and it relies on its heterodimerization partners for activity following ligand binding. The HER2-HER3 heterodimer constitutes a high affinity co-receptor capable of potent mitogenic signaling. HER3 participates in a signaling pathway involved in the proliferation, survival, adhesion, and motility of tumor cells. The HER3 subfamily is part of a larger superfamily that includes other pseudokinases and the the catalytic domains of active kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.