Fitting the kinetic data for FADD DD mutants proved to be complex. All kinetics were measured using intrinsic Trp fluorescence as a probe to allow data to be collected at a low concentration (≤ 1 μM) of protein As for WT, both refolding and unfolding data fit well to a single-exponential equation for all mutants. To avoid the possibility of complication from aggregation events observed in the WT protein, refolding data below 1.5 M urea were not used for fitting the chevron plots. An exception to this rule was made for highly destabilised mutants with a low [urea]50%, where the refolding arm was short (W112A, W148F, L161A and L165A). For S144A, all data below 2.5 M urea were omitted from chevron fitting, due to aggregation. All mutant chevron plots had linear folding arms with essentially the same slope (only three had refolding mkf values that were significantly different from the mean value of 1.7 M- 1).