Calcineurin is an enzyme that dephosphorylates serine and threonine residues in proteins. It is a heterodimer of a 59kDa catalytic A subunit and a 19kDa regulatory B subunit that is activated by the binding of calcium ions and calmodulin. Calcineurin is expressed in many tissues, but its levels are highest in the brain, where it may play a role in learning and memory. It has many substrates, including NFAT, a transcription factor that is activated by dephosphorylation. Complexes of the immuno-suppressants cyclosporine and FK506 with immunophilin proteins such as cyclophilin and FKBP12 are potent and specific inhibitors of Calcineurin activity. Alterations in Calcineurin activity are suspected to play a role in cardiac hypertrophy and graft versus host disease in organ transplantation.