The serine codons UCX and AGY are not interchangeable in the mammalian mitochondrial genomes

The study of silent substitutions in the mammalian mitochondrial genes shows that the probability that two species use the same codon is, as a general rule, equal to the product of the probabilities of use of this codon in the two species. The only appreciable exception is that of serine, for which the UCX and AGY codons are not interchangeable.