Impact of ligand on MTSET inactivation of [3H]5-HT transport in TM6 Cys mutants. Accessibility of Cys mutants was assessed by alteration of [3H]5-HT transport after exposure to MTSET in the absence (black) or presence (gray) of 50 μm 5-HT (A), 50 μm MDMA (B), or 50 μm cocaine (C). Results for each mutant are expressed as the means ± S.E., n = 3, and normalized to C109A. Means of Cys mutants in the presence and absence of substrate were compared with each other using a two-tailed Student's t test, with the asterisks indicating p < 0.05.