INSR is a receptor that binds insulin and has a tyrosine-protein kinase activity. Autophosphorylation activates the kinase activity. This Type I mebrane protein is composed of a tetramer of 2 alpha and 2 beta chains linked by disulfide bonds. The alpha chains contribute to the formation of the ligand-binding domain, while the beta chains carry the kinase domain. After being transported from the endoplasmic reticulum to the Golgi apparatus, the single glycosylated precursor is further glycosylated and then cleaved, followed by its transport to the plasma membrane. Defects in INSR are the cause of insulin resistance of various forms, including mild insulin-resistant diabetes mellitus with acanthosis nigricans, minor physical abnormalities and sometimes polycystic ovaries. Insulin resistance associated with acanthosis nigricans, hirsutism and hyperandrogenism is referred to as insulin resistance type A. Defects in INSR are the cause of Rabson-Mendenhall syndrome, also known as Mendenhall syndrome. It is a severe insulin resistance syndrome characterized by insulin-resistant diabetes mellitus with pineal hyperplasia and somatic abnormalities.