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 2 different calcineurin B isoforms (beta 1 and beta 2) has been reported in rat testis. The catalytic subunit of calcineurin, calcineurin A, isolated from different tissues or different organisms, exhibits some immunological heterogeneity. Further, there are at least 2 isoforms of calcineurin A in bovine brain (alpha 1 and alpha 2, 61 and 59 kDa, respectively).