A quantitative theory of directional mutation pressure proposed in 1962 
explained the wide variation of DNA base composition observed among different 
bacteria and its small heterogeneity within individual bacterial species. The 
theory was based on the assumption that the effect of mutation on a genome is 
not random but has a directionality toward higher or lower guanine-plus-cytosine 
content of DNA, and this pressure generates directional changes more in neutral 
parts of the genome than in functionally significant parts. Now that DNA 
sequence data are available, the theory allows the estimation of the extent of 
neutrality of directional mutation pressure against selection. Newly defined 
parameters were used in the analysis, and two apparently universal constants 
were discovered. Analysis of DNA sequence has revealed that practically all 
organisms are subject to directional mutation pressure. The theory also offers 
plausible explanations for the large heterogeneity in guanine-plus-cytosine 
content among different parts of the vertebrate genome.