The human genome, as in other eukaryotes, has a wide heterogeneity in the DNA 
base composition. The evolutionary basis for this heterogeneity has been 
unknown. A previous study of the human genome (846 genes analyzed) has shown 
that, in the major range of the G+C content in the third codon position 
(0.25-0.75), biases from the Parity Rule 2 (PR2) among the synonymous codons of 
the four-codon amino acids are similar except in the highest G+C range (Sueoka, 
N., 1999. Translation-coupled violation of Parity Rule 2 in human genes is not 
the cause of heterogeneity of the DNA G+C content of third codon position. Gene 
238, 53-58.). PR2 is an intra-strand rule where A=T and G=C are expected when 
there are no biases between the two complementary strands of DNA in mutation and 
selection rates (substitution rates). In this study, 14,026 human genes were 
analyzed. In addition, the third codon positions of two-codon amino acids were 
analyzed. New results show the following: (a) The G+C contents of the third 
codon position of human genes are scattered in the G+C range of 0.22-0.96 in the 
third codon position. (b) The PR2 biases are similar in the range of 0.25-0.75, 
whereas, in the high G+C range (0.75-0.96; 13% of the genes), the PR2-bias 
fingerprints are different from those of the major range. (c) Unlike the PR2 
biases, the G+C contents of the third codon position for both four-codon and 
two-codon amino acids are all correlated almost perfectly with the G+C content 
of the third codon position over the total G+C ranges. These results support the 
notion that the directional mutation pressure, rather than the directional 
selection pressure, is mainly responsible for the heterogeneity of the G+C 
content of the third codon position.