  Because of the base pairing rules in DNA, some mutations experienced by a
portion of DNA during its evolution result in the same substitution, as we can
only observe differences in coupled nucleotides. Then, in the absence of a bias
between the two DNA strands, a model with at most 6 different parameters
instead of 12 is sufficient to study the evolutionary relationship between
homologous sequences derived from a common ancestor. On the other hand the same
symmetry reduces the number of independent observations which can be made. Such
a reduction can in some cases invalidate the calculation of the parameters. A
compromise between biologically acceptable hypotheses and tractability is
introduced and a five parameter reversible no-strand-bias condition (RNSB) is
presented. The identifiability of the parameters under this model is shown by
examples.
