Symmetry and Codon Usage Correlations in the Genetic Code
  The ratios of the codon usage in the quartets and sextets for the vertebrate series exhibit a correlated behaviour which fits naturally in the framework of the crystal basis model of the genetic code. Moreover the observed universal behaviour of these suitably normalized ratios can be easily explained. 
L. Frappat, A. Sciarrino, P. Sorba