The mean ± s.e.m. for τf (A), τs (B) and ratio Af/(Af+As) (C) for WT and all mutant channels. K525Q and R534Q mutants had significantly slower time constants (τf and τs) of deactivation (n = 6) than WT (P < 0.05, ANOVA, followed by Dunnett's method), whereas the R531Q and R537Q mutants had significantly faster time constants (τf and τs) of deactivation than WT (P < 0.05, ANOVA, followed by Dunnett's method). R528Q channels had a faster τf but a slower τs. The fast component of deactivation accounts for over 80% of deactivation for WT as well as all other mutant HERG channels at −130 mV. However, at more depolarized potentials (−80 to −60 mV) the fast component accounts for 50–70% of deactivation for WT compared to 80–90% for R528Q and R537Q, but only 20–30% for R534Q and R531Q.