Ca2+ signaling in nonexcitable cells is typically initiated by receptor-triggered production of inositol-1,4,5-tris-phosphate and the release of Ca2+ from intracellular stores . An elusive signaling process senses the Ca2+ store depletion and triggers the opening of plasma membrane Ca2+ channels [2-5]. The resulting sustained Ca2+ signals are required for many physiological responses, such as T cell activation and differentiation . Here, we monitored receptor-triggered Ca2+ signals in cells transfected with siRNAs against 2,304 human signaling proteins, and we identified two proteins required for Ca2+-store-depletion-mediated Ca2+ influx, STIM1 and STIM2 [7-9]. These proteins have a single transmembrane region with a putative Ca2+ binding domain in the lumen of the endoplasmic reticulum. Ca2+ store depletion led to a rapid translocation of STIM1 into puncta that accumulated near the plasma membrane. Introducing a point mutation in the STIM1 Ca2+ binding domain resulted in prelocalization of the protein in puncta, and this mutant failed to respond to store depletion. Our study suggests that STIM proteins function as Ca2+ store sensors in the signaling pathway connecting Ca2+ store depletion to Ca2+ influx.