Mutations of the insulin receptor gene have been identified in patients with genetic syndromes of insulin resistance associated with acanthosis nigricans. These mutations impair insulin responses by reducing the number of insulin receptors on the surface of target cells, or by reducing the receptor's ability to bind insulin or to undergo insulin-stimulated autophosphorylation, an important step in insulin action. Studies of mutant receptors expressed in transfection systems have contributed to our understanding of the structure-function relationships of the insulin receptor.