Calcineurin, a major soluble calmodulin binding protein in the brain, a Ca2+/calmodulin dependent serine/threonine protein phosphatase, with a relatively narrow substrate specificity. This metalloenzyme, also known as phosphatase 2B, is a heterodimer composed of a calmodulin binding, catalytic alpha subunit 61 kDa, calcineurin A) and calcium binding beta subunit (18 kDa, calcineurin B). The Ca2+ binding subunit, calcineurin B, is immunologically conserved from yeast to mammals. The presence of