Wide variation in the extents of codon usage bias among various organisms. (a) Distribution of CAIave values among 518 prokaryotes with GC content between 35% and 65%. CAIave is indicated as 'Average CAI'. (b) Negative correlation between organisms' CAIave values and their coefficients of variation. (c) Distribution of CAI values among the genes of Syntrophothermus lipocalidus (high CAIave, indicating low extent of codon usage bias). (d) Distribution of CAI values among the genes of Vibrio vulnificus (low CAIave, indicating high extent of codon usage bias). CAI: Codon Adaptation Index; GC: genome content.