Recent RNA interference screens have identified several proteins that are essential for store-operated Ca2+ influx and Ca2+ release-activated Ca2+ (CRAC) channel activity in Drosophila and in mammals, including the transmembrane proteins Stim (stromal interaction molecule) and Orai. Stim probably functions as a sensor of luminal Ca2+ content and triggers activation of CRAC channels in the surface membrane after Ca2+ store depletion. Among three human homologues of Orai (also known as olf186-F), ORAI1 on chromosome 12 was found to be mutated in patients with severe combined immunodeficiency disease, and expression of wild-type Orai1 restored Ca2+ influx and CRAC channel activity in patient T cells. The overexpression of Stim and Orai together markedly increases CRAC current. However, it is not yet clear whether Stim or Orai actually forms the CRAC channel, or whether their expression simply limits CRAC channel activity mediated by a different channel-forming subunit. Here we show that interaction between wild-type Stim and Orai, assessed by co-immunoprecipitation, is greatly enhanced after treatment with thapsigargin to induce Ca2+ store depletion. By site-directed mutagenesis, we show that a point mutation from glutamate to aspartate at position 180 in the conserved S1-S2 loop of Orai transforms the ion selectivity properties of CRAC current from being Ca2+-selective with inward rectification to being selective for monovalent cations and outwardly rectifying. A charge-neutralizing mutation at the same position (glutamate to alanine) acts as a dominant-negative non-conducting subunit. Other charge-neutralizing mutants in the same loop express large inwardly rectifying CRAC current, and two of these exhibit reduced sensitivity to the channel blocker Gd3+. These results indicate that Orai itself forms the Ca2+-selectivity filter of the CRAC channel.