Figure 4. Lack of evidence for major codon usage biases.

Correlation between GC content at third codon position (GC₃) and the effective codon usage statistic (ENC) among (a) large DNA viruses infecting invertebrate hosts and (b) large DNA viruses infecting vertebrate hosts. The black line represents the expected ENC values (ENC*) calculated based on GC content. Most ENC values in both groups of viruses are on, or just below the ENC* values suggesting the absence of strong translational selection.