Effects of internal MTS reagents on cysteine-substituted CFTR-TM1 mutants. Mean effect of treatment with 200 µM MTSES (left) or 2 mM MTSET (right) on macroscopic current amplitude in cys-less CFTR and in each of 20 different cysteine-substituted TM1 mutants. Note that no currents were recorded from patches excised from cells transfected with E92C cDNA (see Results). Effects of these two MTS reagents were quantified by measuring current amplitudes at membrane potentials of +80 mV (for MTSES, left) and −80 mV (for MTSET, right) before MTS reagent application and after complete modification had taken place. Mean of data from three to nine patches. Asterisks indicate a significant difference from cys-less (P < 0.05).