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Fig. 2 nkx2.2a, nkx2.2b and nkx2.9 are expressed in overlapping domains. (A-I) Head (A,D,G) and trunk (B,E,H lateral view; C,F,I transverse section) of a 24 hpf zebrafish embryo hybridized to nkx2.2a (A-C), nkx2.2b (D-F) and nkx2.9 (G-I) antisense probes. Whereas expression of nkx2.2a (A) along the ventral brain is continuous, the expression domains of nkx2.2b and nkx.2.9 show a gap (D,G, asterisk) in the prethalamic and thalamic areas adjacent to the zona limitans intrathalamica. All three genes are expressed in the lateral floor plate of the trunk. In the spinal cord (B,E,H), the expression of nkx2.2a and nkx2.9 is discontinuous, with gaps of non-expressing cells (B,H, black arrowheads), whereas all cells of the lateral floor plate appear to express nkx2.2b (E). All three genes are restricted to the lateral floor plate (C,F,I). Orientation of whole-mount embryos (A,B,D,E,G,H): anterior left, dorsal up. Lateral views of the head (A,D,G) and the spinal cord are at the level of the hindgut extension. Transverse sections (C,F,I) are at the level of the hindgut extension, dorsal up. m, medial floor plate; n, notochord. Scale bar: 100 μm in A,D,G; 25 μm in B,C,E,F,H,I.