**2. Evolution of the structural characteristics of the GLUT4 gene**

Among the sequenced genomes of vertebrate species, the GLUT4 gene (named SLC2A4) is found in mammals (41 species), reptiles (2 species) and ray-finned fishes (10 species) (Figure 1), suggesting that the GLUT4 gene has been conserved throughout evolution. Surprising‐ ly, the GLUT4 gene is absent among birds and amphibians. The lack of GLUT4 in birds, but the presence of other GLUT family members (Glut1, Glut2 and Glut3), has recently been confirmed experimentally [27]. Moreover, no GLUT4 gene is present in the only amphib‐ ian genome available to date (i.e. *Xenopus tropicalis*) and searches for GLUT4 in Xenopus EST databases were unable to yield any transcript with homology for GLUT4. At this time, it is not known if the lack of the GLUT4 gene is general of the avian and amphibian classes, particularly since only a very reduced number of species have been examined, and, more importantly, if these two groups of vertebrates have subsequently lost the GLUT4 gene after their emergence.

**Figure 1.** Evolutionary tree of the GLUT4 gene (SLC2A4) in vertebrates. Alignment of the different GLUT4 gene se‐ quences is shown on the right. White sections of the alignment indicate gaps in the sequence.
