Calcineurin (PP2B), a serine/threonine phosphatase controlled by cellular calcium, was
originally identified in extracts of mammalian brain. Within the past few years this
phosphatase has been implicated in a wide variety of biological responses including
lymphocyte activation, neuronal and muscle development, neurite outgrowth, and
morphogenesis of vertebrate heart valves. Progress in this area has been greatly
accelerated by the drugs cyclosporin A and FK506, which use a unique mechanism of action …