Copyright 2006, The Endocrine Society. Schematic representation of the RET tyrosine kinase. The extracellular region comprises four cadherin domains and a cysteine rich domain. A single transmembrane region spans the cell membrane, and the two tyrosine kinase domains (TK1 and TK2) are located in the intracellular region. There are three isoforms of RET (RET9, RET43, RET 51) which are indicated. aa, Amino acids. [Adapted from De Groot et al. ].