Catalytic domain of the Protein Tyrosine Kinase, Fibroblast Growth Factor Receptor 4PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Unlike other FGFRs, there is only one splice form of FGFR4. It binds FGF1, FGF2, FGF6, FGF19, and FGF23. FGF19 is a selective ligand for FGFR4. Although disruption of FGFR4 in mice causes no obvious phenotype, in vivo inhibition of FGFR4 in cultured skeletal muscle cells resulted in an arrest of muscle progenitor differentiation. FGF6 and FGFR4 are uniquely expressed in myofibers and satellite cells. FGF6/FGFR4 signaling appears to play a key role in the regulation of muscle regeneration. A polymorphism in FGFR4 is found in head and neck squamous cell carcinoma. FGFR4 is part of the FGFR subfamily, which are receptor PTKs (RTKs) containing an extracellular ligand-binding region with three immunoglobulin-like domains, a transmembrane segment, and an intracellular catalytic domain. The binding of FGFRs to their ligands, the FGFs, results in receptor dimerization and activation, and intracellular signaling. The binding of FGFs to FGFRs is promiscuous, in that a receptor may be activated by several ligands and a ligand may bind to more that one type of receptor. The FGFR4 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.