Calmodulin (CaM) is a small acidic protein that contains four EF-hand motifs, each of which can bind a calcium ion, therefore it can bind up to four calcium ions. The protein has two approximately symmetrical domains, separated by a flexible hinge region. Calmodulin is the prototypical example of the EF-hand family of Ca2+-sensing proteins. Changes in intracellular Ca2+ concentration regulate calmodulin in three distinct ways. First, by directing its subcellular distribution. Second, by promoting association with different target proteins. Third, by directing a variety of conformational states in calmodulin that result in target-specific activation. Calmodulin binds and activates several effector protein (e.g. the CaM-dependent adenylyl cyclases, phosphodiesterases, protein kinases and the protein phosphatase calcineurin).