The PKC family of serine⁄threonine kinases, including PRKCI (PKC iota), is activated intracellularly by signal transduction pathways. In humans, at least 12 different PKC polypeptides have been identified. These isoforms differ in primary structure, tissue distribution, subcellular localization, mode of action in vitro, response to extracellular signals, and substrate specificity. PKC alpha, beta I, beta II and gamma form the conventional family; their activities are Ca2+- and phospholipid-dependent.