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.