INSR, or Insulin Receptor, is a transmembrane receptor protein that plays a key role in mediating the biological effects of insulin, a hormone involved in regulating glucose metabolism, lipid metabolism, and cellular growth. INSR is primarily located on the surface of target cells, such as adipocytes, hepatocytes, and skeletal muscle cells. Its main function is in insulin signaling pathways — when insulin binds to the extracellular domain of INSR, the receptor undergoes conformational changes that trigger autophosphorylation of tyrosine residues in its intracellular domain, activating its tyrosine kinase activity.