The Insulin-like Growth Factor 1 Receptor (IGF1R) is also known as CD221 and JTK13, and is a transmembrane receptor that is activated by IGF-1 and by the related growth factor IGF-2. It belongs to the large class of tyrosine kinase receptors. This receptor mediates the effects of IGF-1, which is a polypeptide hormone similar in molecular structure to insulin. IGF1R is made up of two α subunits and two β subunits. Both the α and β subunits are synthesized from a single mRNA precursor. The precursor is then glycosylated, proteolytically cleaved, and crosslinked by disulfide bonds to form a functional transmembrane αβ chain. The α chains are located extracellularly, while the β subunits span the membrane and are responsible for intracellular signal transduction upon ligand stimulation. IGF1R has a binding site for ATP, which is used to provide the phosphates for autophosphorylation. There is 60% sequence homology between IGF1R and the insulin receptor. In response to ligand binding, the α chains induce the tyrosine autophosphorylation of the β chains. This event triggers a cascade of intracellular signaling that, while somewhat cell type specific, often promotes cell survival and cell proliferation.