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.