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Fig. 1 osr2 expression during zebrafish development. A: At 11 hpf, osr2 is expressed in the posterior embryonic midline (tilted lateral view, anterior to the left). B: Cross-section of an 11-hpf embryo at the level of the dotted line indicated in A showing osr2 (blue arrowhead) expression in floorplate (fp) cells overlying the notochord marked by red ntl expression (black arrowheads). C: Expression of osr2 is biased to cells on the right side of the forming floorplate. Black arrowheads mark the boundaries of the notochord. L, left; R, right. D: At 24 hpf, osr2 is expressed in the pronephric proximal tubule and gut. E, F: Cross-section of 24 hpf embryo indicated in Figure 1D showing osr2 expression in the proximal pronephros (E) and gut (F; red arrow). Distal tubules of the pronephros do not express osr2 (dotted circles). osr2 expression is also observed in mesenchyme cells (black arrowheads) ventral and lateral to the pronephros. G: At 48 hpf, osr2 is expressed in fin (red arrow), pronephric proximal tubule (white arrowhead), posterior gut (black arrowhead), and in gut mesenchyme (red arrowhead) at the level for the pectoral fin. H, I: Cross-section of the embryo in G at the level indicated showing expression of osr2 in pectoral fin mesenchyme (H) pronephric tubules, and gut mesenchyme (I; red arrowhead). J: Double in situ hybridization showing gut mesenchyme (red arrowhead) expressing osr2 (blue) anterior to the exocrine pancreas as indicated by insulin (red) expression at 48 hpf. K: Gut mesenchyme (red arrowhead) expressing osr2 (blue) was adjacent to the pneumatic duct as indicated by wnt2 (red) expression at 55 hpf.