Catalytic domain of the Protein Tyrosine Kinase, REarranged during Transfection proteinPTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. RET is a receptor PTK (RTK) containing an extracellular region with four cadherin-like repeats, a calcium-binding site, and a cysteine-rich domain, a transmembrane segment, and an intracellular catalytic domain. It is part of a multisubunit complex that binds glial-derived neurotropic factor (GDNF) family ligands (GFLs) including GDNF, neurturin, artemin, and persephin. GFLs bind RET along with four GPI-anchored coreceptors, bringing two RET molecules together, leading to autophosphorylation, activation, and intracellular signaling. RET is essential for the development of the sympathetic, parasympathetic and enteric nervous systems, and the kidney. RET disruption by germline mutations causes diseases in humans including congenital aganglionosis of the gastrointestinal tract (Hirschsprung's disease) and three related inherited cancers: multiple endocrine neoplasia type 2A (MEN2A), MEN2B, and familial medullary thyroid carcinoma. The RET 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.