Deletion of both STIM proteins diminishes the overall IP3-mediated response. (A) Average intracellular Ca²⁺ responses following application of Ca²⁺-free solution containing 200 μM ATP in a representative experiment are shown for WT (n = 48) and dKO (n = 46) cells. (B) Bars show the mean ± SEM of the percentage of responding cells (gray bars) or quantitation of ATP-induced Ca²⁺ release (AUC, purple bars) from three experiments as shown in A. (C) Cells were loaded with ci-IP3 and Cal-590 (see Materials and methods). Panel shows the average intracellular Ca²⁺ responses for WT (n = 163), S1KO (n = 139), S2KO (n = 200), or dKO (n = 189) cells following a brief (1-s) illumination with 405-nm laser. Note the lack of response in WT cells lacking ci-IP3 (n = 19). (D) Bars show the mean ± SEM of the percentage of responding cells (gray bars) or quantitation of UV-induced Ca²⁺ release (AUC, purple bars) from seven experiments as shown in C. Statistics: two-tailed t test (B) and one-way ANOVA with Tukey’s post hoc test (D); *P < 0.05, **P < 0.01, ***P < 0.001. AUC, area under the curve.