Within the last 30 years of research the biological roles of calcineurin have progressed from a putative inhibitor of the calmodulin dependent phosphodiesterase to the ground-breaking discovery that it is the target of immunosuppressant drugs such as cyclosporin A. As a serine/threonine protein phosphatase, calcineurin participates in a number of cellular processes and Ca2+ dependent signal transduction pathways. Calcineurin plays a key role in regulating the transcription factor NF-AT during T-cell activation, and in mediating responses of microorganisms to cation stress. Recently, this protein was studied extensively in relation to diseases such as schizophrenia and diabetes.