In cells of the immune system that are stimulated by antigen or antigen-antibody 
complexes, Ca(2+) entry from the extracellular medium is driven by depletion of 
endoplasmic reticulum Ca(2+) stores and occurs through specialized 
store-operated Ca(2+) channels known as Ca(2+)-release-activated Ca(2+) (CRAC) 
channels. The process of store-operated Ca(2+) influx is essential for 
short-term as well as long-term responses by immune-system cells. Short-term 
responses include mast cell degranulation and killing of target cells by 
effector cytolytic T cells, whereas long-term responses typically involve 
changes in gene transcription and include T and B cell proliferation and 
differentiation. Transcription downstream of Ca(2+) influx is in large part 
funneled through the transcription factor nuclear factor of activated T cells 
(NFAT), a heavily phosphorylated protein that is cytoplasmic in resting cells, 
but that enters the nucleus when dephosphorylated by the calmodulin-dependent 
serine/threonine phosphatase calcineurin. The importance of the 
Ca(2+)/calcineurin/NFAT signalling pathway for lymphocyte activation is 
underscored by the finding that the underlying defect in a family with a 
hereditary severe combined immune deficiency (SCID) syndrome is a defect in CRAC 
channel function, store-operated Ca(2+) entry, NFAT activation and transcription 
of cytokines, chemokines and many other NFAT target genes whose transcription is 
essential for productive immune defence. We recently used a two-pronged genetic 
approach to identify Orai1 as the pore subunit of the CRAC channel. On the one 
hand, we initiated a positional cloning approach in which we utilised 
genome-wide single nucleotide polymorphism (SNP) mapping to identify the genomic 
region linked to the mutant gene in the SCID family described above. In 
parallel, we used a genome-wide RNAi screen in Drosophila to identify critical 
regulators of NFAT nuclear translocation and store-operated Ca(2+) entry. These 
approaches, together with subsequent mutational and electrophysiological 
analyses, converged to identify human Orai1 as a pore subunit of the CRAC 
channel and as the gene product mutated in the SCID patients.