Calcineurin signaling has been implicated in a broad spectrum of developmental 
processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, 
calcium-activated protein phosphatase composed of catalytic and regulatory 
subunits. The serine/threonine-specific phosphatase functions within a signal 
transduction pathway that regulates gene expression and biological responses in 
many developmentally important cell types. Calcineurin signaling was first 
defined in T lymphocytes as a regulator of nuclear factor of activated T cells 
(NFAT) transcription factor nuclear translocation and activation. Recent studies 
have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in 
cardiovascular and skeletal muscle development in vertebrates. Inhibition, 
mutation, or forced expression of calcineurin pathway genes result in defects or 
alterations in cardiomyocyte maturation, heart valve formation, vascular 
development, skeletal muscle differentiation and fiber-type switching, and 
cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found 
in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic 
studies have demonstrated specific myogenic functions for the phosphatase in 
their development. The ability to investigate calcineurin signaling pathways in 
vertebrates and model genetic organisms provides a great potential to more fully 
comprehend the functions of calcineurin and its interacting genes in heart, 
blood vessel, and muscle development.