Selective constraints deduced from distances in proximal promoter and enhancer. Branch lengths derived from the evolutionary divergence of the proximal promoter (1 kb), the alignable enhancer and the fifth intron of the NOS-2 gene were compared for each one of the species. Two alternative and comparable evolutionary schemes using human-mouse-cow and macaque-rat-dog species were used. Branch length to species i (Li) was derived from pairwise distances (Di-j) according to an exact equation (see methods). See text for further explanation.