Response function of Orai1-GCaMP6f. (A and B) Residual plots for nonlinear curve fitting to the Hill equation (A; R² = 0.992) and linear regression (B; R² = 0.978) for the scatterplots of whole-cell current versus fluorescence shown in Fig. 4, C and D. (C and D) Scatterplots of normalized whole-cell current versus fluorescence for steps to increasingly negative membrane potentials (−10, −40, −70, and −100 mV; cells A, E, and F; red diamonds) and for individual test pulses to negative membrane potentials (−10, −40, −70, and −100 mV; cells A–F; blue diamonds). Lines indicate fitting to the Hill equation for a stepped test pulse (light red; R² = 0.993, hc = 1.7 with 95% CI of 1.3–2.3) and for individual test pulses (light blue; R² = 0.957, hc = 1.7 with 95% CI of 1.1–2.7). Note the high degree of overlap with similar maximum fluorescence values for the two fitted curves. (D) Enlargement of lower central portion of C. Whole-cell recordings were performed in 2 mM external Ca²⁺.