With the determination of the first gene sequences, it was noticed that alternative synonymous codons are not used in equal frequencies. Grantham and colleagues pioneered the compilation of codon usage data for many genes, 1 , 2 allowing the description of codon usage frequency patterns, and the use of multivariate statistics to elucidate variations in these patterns. As the gene sequence database has expanded, it has become clear that synonymous codon usage varies both among genes from one genome, and between species. 1 4 More recently, a firm theoretical basis for these observed patterns has been established, in terms of the effects of natural selection, biased mutation, and genetic drift. 5 7 Most of this work has focused on sense codons, and rather little attention has been paid to termination (or “stop”) codons.