The GC-content of bacteria varies in a consistent manner across types of sites (reproduced from Hershberg, R., Petrov, D.A., 2010. Evidence that mutation is universally biased towards AT in bacteria. PLOS Genetics 6, e1001115). GC-content of nonsynonymous (red) and synonymous (blue) sites are plotted against the GC-contents of intergenic regions from the same bacteria. Each point represents a different fully sequenced bacterial genome.