The three-base periodicity usually found in exons has been used for several purposes, as for example the prediction of potential genes. In this paper, we use a data model, previously proposed for encoding protein-coding regions of DNA sequences, to build signatures capable of supporting the construction of meaningful dendograms. The model relies on the three-base periodicity and provides an estimate of the entropy associated with each of the three bases of the codons. We observe that the three entropy values vary among themselves and also from species to species. Moreover, we provide evidence that this makes it possible to associate a three-state entropy vector with each species, and we show that similar species are characterized by similar three-state entropy vectors.