The genome of higher eukaryotes consists of genes having a widely heterogeneous 
base composition at the third codon position. Ubiquitous variability of the DNA 
base composition has the following two aspects: intragenomic heterogeneity of 
the G+C content and the amino-acid-specific translation-coupled biases from the 
Parity Rule 2 (PR2). PR2 is an intrastrand rule where A = T and G = C are 
expected if there is no bias in mutation and selection between the two 
complementary strands of DNA. To examine whether or not the biases from PR2 are 
responsible for the wide heterogeneity of the DNA G+C content in human, the 
third codon position of 846 human genes was analyzed. Genes were separated into 
six groups according to their G+C content of the third codon position, and each 
group was examined for the translation-coupled PR2 biases in the nucleotide 
composition of the third codon position for two- and four-codon amino acids. The 
results show that genes in the different G+C content groups have similar PR2 
biases, indicating that the intragenomic heterogeneity of the G+C content is not 
correlated with translation-coupled biases from the PR2. Therefore, the 
heterogeneity of the G+C content is likely to be determined by some other 
mechanism (e.g. locally variable directional mutation pressures) than 
amino-acid-specific selections for the codon preference.