The codon responseto genome GC content varies with position. A re-plot of GC3 versus GC1, GC2 from , using the additional sequence data now available. Each point represents an organism, classified by domain: archaea, gray; bacteria, black; eukaryotes, white. GC1, diamonds; GC2, squares. Lines are model I least-squares regressions. Where GC3 = 0%, the remaining %GC in position 1 and position 2 is assumed to represent constant sites (that is, those fixed by selection to remain G or C). Similarly, where GC3 = 100%, the remaining %AT in position 1 and position 2 is assumed to represent constant sites where A or T have been fixed.