Catalytic domain of the Protein Tyrosine Kinase, HER4PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. HER4 (ErbB4) 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. Ligands that bind HER4 fall into two groups, the neuregulins (or heregulins) and some EGFR (HER1) ligands including betacellulin, HBEGF, and epiregulin. All four neuregulins (NRG1-4) interact with HER4. Upon ligand binding, HER4 forms homo- or heterodimers with other HER proteins. HER4 is essential in embryonic development. It is implicated in mammary gland, cardiac, and neural development. As a postsynaptic receptor of NRG1, HER4 plays an important role in synaptic plasticity and maturation. The impairment of NRG1/HER4 signaling may contribute to schizophrenia. The HER4 subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.