The most avoided and the most preferred codon pairs are uniformly distributed in all three domains of life. The map is clustered on the basis of the conservation of avoidance and preference of different codon pairs. The avoided codon pairs are marked in yellow (obs/exp < 0). The preferred codon pairs are marked in blue (obs/exp > 0). Codon pairs without bias are black (obs/exp = 0). No additional criteria were applied to the figure. Codon pairs are ranked downwards according to the decreasing conservation of avoided codon pairs in the organisms studied. B – bacteria, A – archaea, E – eukaryotes.