Sections of four current records with different K⁺/Tl⁺ mixtures on the luminal side (pipette). [Tl⁺] is given at the traces with [K⁺] = 150 mM − [Tl⁺]. Cytosolic solution contained 150 mM K⁺. Membrane potential was −100 mV for all records. Channel openings result in downward deflections. With increasing Tl⁺ concentration, the open-channel noise increases and the apparent current decreases, indicating the increase of unresolved gating events. The upper horizontal lines give the baseline (closed channel). The dotted lines show the apparent current I_app,∞ taken from the plateau in Fig. 3. I_app,1 obtained from fit-by-eye (Riessner et al., 2002) is presented by solid horizontal lines.