KA/KS differences by disease system. Significance of KA/KS differences determined by Wilcoxon analysis indicate that both rat (color) and mouse (grey) demonstrate significantly strong purifying selection for the orthologs of the neurological and malformation-syndrome disease categories. The immunological, hematological, and pulmonary disease systems demonstrated significantly lower conservation. Disease categories are listed along the vertical axis in the order of standardized score from low to high. The P value is indicated above the bars in the figure; P values of less than 0.05 are considered statistically significant.