Mutability plots. Mutability plots were determined as described in the text (grey bars). The mutability of individual residues was averaged over a window of 1, 5, 11 or 25 codons. The hypo- or hypermutability was then plotted as a bar in the center of the specified window and the window was shifted downstream one codon at a time. Individual hypomutable regions, designated A, B, C, and D are indicated on the plot for the 11 codon window. For comparison, the difference between mutability calculated by previous methods (5) and mutability as described in this manuscript is also shown (circles).