Region-independent chi-square analysis. Three-dimensional plot illustrating χ2 of each significantly hypomutable region plotted against region length and amino acid residue number. Calculations were performed as described in the text. Only regions that are significant at the 0.005 level are plotted; the whole window is plotted whenever this significance level is achieved. If a residue is involved in more than one significant region of the same length, the region with the highest χ2 value is plotted. Colours indicate χ2 of the region and range from deep red (χ2 > 15, corresponding to α < 0.0001) to pale green (χ2 > 8, α < 0.005). Four hypomutable regions approximating regions A-D (Figure 1) are evident.