Nonrandom CpG mutations affect the synonymous codon usage of moderately GC-rich single copy actin genes

In species where actin genes exist as single copies, analysis of their synonymous codon usage and of the substitutions occurring between the genes of closely related species shows that there is a positive selection for codons that do not have highly mutable CpG dinucleotides in codon positions 2 and 3 when the GC content of these genes is less than 57%.