Fig. 7. A schematic model of wing/leg development. Turquoise and pink rectangles in the diagrams of the embryos represent the presumptive wing and leg region, respectively. The size of a letter indicates the degree of gene expression. In the presumptive wing region, wing-type identity is specified and determined before stage 9. This determination is maintained until stage 13, and this region begins to exhibit a wing phenotype, as Tbx5 expression, at stage 13. In this process, the abilities to induce Tbx5 and to repress Tbx4 in the presumptive wing level-median tissues contribute to determination and maintenance of wing-type identity. In the presumptive leg region, leg-type identity is specified before stage 9 and determined at stage 12/13, and this region soon begins to differentiate into a leg bud with Tbx4 up-regulation at stage 13. Tbx5-repressing and Tbx4-inducing abilities in the presumptive leg level-median tissues contribute to the determination of leg-type identity.