The Nc-plot Provides a powerful graphical method of multivariate analysis Many different variables could be plotted along the x-axis, but GC3 is the predominant one (GC3 = G + C composition at the third position, should reflect genome-wide composition bias) Can (loosely) test hypotheses about codon bias, specifically whether the observed pattern is probably due to composition bias or selection on synonymous codons x = % G+C