  Although several synonymous codons can encode the same aminoacid, this
symmetry is generally broken in natural genetic systems. In this article, we
show that the symmetry breaking can result from selective pressures due to the
violation of the synonym symmetry by mutation and recombination. We conjecture
that this enhances the probability to produce mutants that are well-adapted to
the current environment. Evidence is found in the codon frequencies of the HIV
{\it env} protein: the codons most likely to mutate and lead to new viruses
resistant to the current immunological attack, are found with a greater
frequency than their less mutable synonyms.
