(A, F, K) Inward unitary currents of the cut-ΔR(D1370N) CFTR background construct, and of channels bearing mutations at position 172 (A), 961 (F), or 1068 (K), in the same background. Letters to the left of the traces indicate the amino acid present in the target position; the native residue is marked by red. Currents were recorded at −80 mV, in symmetrical 140 mM Cl-; dashes on the left mark zero-current level. (B–D, G-I) and (L–N), Mean burst (B, G, L, τb) and interburst (C, H, M, τib) durations and open probabilities (D, I, N, Po) of the constructs in A, F, and K, respectively. Red horizontal lines highlight the respective control values of the background construct which is identified by the red letter representing the native target residue. All data are shown as mean ± SEM (n = 3–8 for data in (B–D), n = 4–15 for data in G-I, n = 4–7 for data in L-N). (E, J, O) Brønsted plots for position 172 (E), 961 (J), and 1068 (O). Red symbol and letter identifies the background construct. Solid lines are linear regression fits with slope Φ indicated.