  It has been argued that the sum of usage probabilities for codons, belonging
to quartets, that have as third nucleotide C or A, is independent of the
biological species for vertebrates. The comparison between the theoretical
correlation matrix derived from these sum rules and the experimentally computed
matrix for 26 species shows a satisfactory agreement. The Shannon entropy,
weakly depending on the biological species, gives further support. Suppression
of codons containing the dinucleotides CG or AU is put in evidence.
