Number of CUG codons in the reference data. The total number of CUG positions for every set of reference proteins is shown together with the numbers of CUG positions conserved in at least two and five genes. To account for different protein lengths (e.g. 200 amino acids in dynactin3 p24 proteins compared to up to 4,000 amino acids in dynein heavy chain proteins), the total number of CUG positions per 1,000 amino acids is also plotted showing that CUG codons are not particularly enriched in certain protein families. Values for all species using standard codon usage (left side) are contrasted with those for all species using alternative yeast codon usage (right side). Detailed numbers are available in Additional file 3.