Fly SO, Xenopus Six1 and Xenopus Six2 proteins have a high level of amino acid identity (82.5%; black boxes) throughout their Six domain (SD, aa 1-120), the protein-protein interaction domain. Only 4 amino acid substitutions are non-conserved (arrowheads). All other changes in sequence involve conserved (:) or semi conserved (.) amino acids. The alignment was performed using Clustal W version 2.0 (Larkin et al., 2007). Dm, Drosophila melanogaster; Xl, Xenopus laevis; Xt, Xenopus tropicalis.