Calcineurin (also called protein phosphatase-2B) is a calmodulin-regulated protein phosphatase which plays an important role in signal transduction. The enzyme is a heterodimer of a 58-59 kDa calmodulin-binding catalytic subunit (calcineurin A) and a small (i.e. 19 kDa) Ca(2+)-binding regulatory subunit (calcineurin B). The highly conserved calcineurin B is encoded by a single gene in all tissues except testes, whereas there are three isoforms of calcineurin A (alpha, beta and gamma) encoded by genes on three different chromosomes. This enzyme can play a critical role in transcriptional regulation and growth control in T lymphocytes by a mechanism believed to involve dephosphorylation of the nuclear factor NF-AT which is essential for transcription of the interleukin-2 gene. To better evaluate the potential role of the calcineurin genes in human genetic disorders, we have studied their chromosome locations. Calcineurin B (PPP3R1) is located on human chromosome 2p16-->p15 and calcineurin A beta (PPP3CB, previous gene symbol CALNB) is present on 10q21-->q22. We confirm the localization of calcineurin A alpha (PPP3CA, previous gene symbol CALNA) to chromosome 4 without regional localization.