Significant increase of GC content before CGN codons.

The datasets used are homologous fragments from the species pairs (see Table 1 for details). We counted A/T to G/C substitutions and G/C to A/T substitutions in fish genes. Particularly, the substitutions before conserved CGN codon and all the remaining silent codon positions were detected in the homologous fragments of the fish genes. Fisher's exact test was used to evaluate the difference in substitutions of A/T to G/C and G/C to A/T before the CGN codon.