Synonymous codon bias is not caused by mutation bias in G+C-rich genes in humans

it is has been suggested that synonymous codon bias is a consequence of mut ation bias in mammals. We tested this hypothesis in humans using single-nuc leotide polymorphism data. We found a pattern of polymorphism which was inc onsistent with the mutation bias hypothesis in G+C-rich genes. However, the data were consistent with the action of natural selection or biased gene c onversion. Similar patterns of polymorphism were also observed in noncoding DNA, suggesting that natural selection or biased gene conversion may affec t large tracts of the human genome.