Disclosed are methods of identifying an agent that modulates an NFAT regulator protein. One such method comprises contacting at least one test agent with a recombinant cell comprising at least one NFAT regulator protein or fragment or derivative thereof, assessing the effect of the test agent on an activity, interaction, expression, or binding to the NFAT regulator protein or fragment or derivative thereof, and identifying the test agent that has an effect on an activity, interaction, expression, or binding to the NFAT regulator protein or fragment or derivative thereof, whereby the identified test agent is characterized as an agent that modulates an NFAT regulator protein. Methods of identifying an agent that modulates intracellular calcium, methods to screen for an agent that modulates NFAT regulator function, methods to diagnose unexplained immunodeficiency in a subject, and methods for identifying an agent for treating or preventing a disease or disorder associated with a NFAT regulator protein or calcium signaling are also disclosed.