Calcineurin is a calcium/calmodulin-dependent serine/threonine protein phosphatase. The active enzyme is a heterodimer consisting of a catalytic A subunit of 61 kDa and a myristylated regulatory B subunit of 19 kDa that binds calcium. The catalytic A subunit consists of an amino terminal catalytic domain, followed by calmodulin and B subunit binding sites, and a caboxyl-terminal autoinhibitory domain. Calnineurin is the target of immunosuppressant drμgs such as cyclosporin A and FK506. Calcineurin controls the translocation of nuclear factors of activated T cells (NFAT), a family of transcriptional activators that control the expression of cytokine genes essential to immune response, from the cytosol into the nucleus of activated T cells. Calcineurin has been proposed to play an important role in mediating calcium-activated cell death.