Family tree of relatedness of homoeo domains. Pairwise comparisons of the protein sequences of the homoeo domains encoded by Drosophila genes (Antp, Ubx, ftz, en and er), yeast genes (a1 and α2) and sequences isolated by homology from humans (Hu1 and Hu2) and frogs (AC1) were used to score divergence. The comparison was confined to residues 29-58 because this region is well conserved among all of these sequences. To produce homology scores we gave one point for amino-acid identity and half a point for similarity. The data are assembled into a family tree by indicating the divergence of each sequence that approximates all the pairwise comparisons. The sequence data are taken from Levine et al28 (Hu1 and Hu2), Scott and Weiner26 (Ubx and ftz), McGinnis et a125 (Antp), Poole et al.35 (en and er), Carraso et al.30 (AC1), and Tatchell et al.32 (α2 and a1). Note that the discrepancies in the published sequence25,26 for Ubx are significant in comparison to the divergence among class I homoeo domains.