FVIII stability of K1813A, K1818A, and K1813A/K1818A mutants. FVIII WT or mutant (0.5 nM) were incubated at 55°C before measuring FVIII activity at the indicated times in a 1-stage clotting assay. The symbols used are defined in Figure 1. FVIII activity was expressed as the fold change of the initial value and was plotted as a function of incubation time. The experiments were performed 3 times, and the average and standard deviation calculations are shown. The data were fitted to the formula given in Equation 2.