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Fig. 2 The expression of sall4 in the pectoral fins is regulated by tbx5 but not fgf24. (A) Schematic representation of tbx5, fgf24, sall4 and fgf10 expression during early stages of pectoral fin development. Expression of tbx5 initiates at 17 hpf, fgf24 at 18 hpf, sall4 at 22 hpf and fgf10 at 24 hpf. The genes are all initially expressed throughout the mesenchyme of the pectoral fin primordia. At the time fin bud outgrowth is initiated, fgf24 expression becomes downregulated in the mesenchyme and simultaneously upregulated in the overlying ectoderm. Dorsal (B-D) and dorsolateral (E,F) views of embryos, with red arrows and brackets highlighting the pectoral fin primordia. sall4 is expressed in wild-type pectoral fin primordia at 26 hpf (B), but it is absent in the primordia of embryos injected with 4 ng of tbx5 MO (C). Although sall4 expression is absent in the pectoral fin primordia, it continues to be expressed normally in other regions of tbx5 morphant embryos (black arrowheads in B,C). (D) tbx5 expression in an embryo injected with 4 ng of tbx5 MO at 26 hpf. (E) sall4 is still expressed in the pectoral fin primordia of embryos injected with 9 ng of fgf24 MO, although the domain of expression is more dispersed, consistent with a loss of fgf24 function. This expression pattern is similar to that of tbx5 in embryos injected with 9 ng of fgf24 MO (F).