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Fig. 5 Hoxa13b-binding elements regulate tailbud expression. (A) Schematic of constructs used to make transgenic fish. The top line shows the position of the various elements in the zebrafish genome. The same elements from the catfish tbxta gene, shown with their location relative to the normal start of tbxta transcription, were also placed in front of GFP. (B-E) GFP expression in transgenic embryos at the gastrula and 20-somite stages. The bottom panels show a magnified view of the tailbud, focused on the midline. In embryos expressing the proximal 2.1?kb promoter only, expression is strong in the gastrula and later in the notochord (arrow), but absent from NMps (arrowhead) and mesoderm (B). With upstream elements added to the proximal promoter, or with the 5.7?kb catfish promoter, expression is strong in the gastrula stage and during somitogenesis, particularly in the somites (C-E). Some expression is observed in the neural tube because GFP expressed in the NMps perdures. Fig. S5 shows a closeup of expression in the posterior somites and a comparison with marker gene expression in the NMps.