Fer [feline encephalitis virus (FES)-related kinase], also called Tyk3, is a widely expressed 822 amino acid, 94 kDa intracellular non-receptor tyrosine kinase of the FES/FPS family. Fer is found both in the cytoplasm and the nucleus, and interacts with several transmembrane receptors, cytosolic proteins, and nuclear chromatin. Through its interactions and kinase activity, Fer/Tyk3 influences synapse formation, cytoskeletal rearrangement, cell survival and cell cycle progression.