  In this note, we characterize the embeddability of generic Kimura 3ST Markov
matrices in terms of their eigenvalues. As a consequence, we are able to
compute the volume of such matrices relative to the volume of all Markov
matrices within the model. We also provide examples showing that, in general,
mutation rates are not identifiable from substitution probabilities. These
examples also illustrate that symmetries between mutation probabilities do not
necessarily arise from symmetries between the corresponding mutation rates.
