Fig. 6. The homeodomain binding-sites in homology region-c (HR-c) are required at the onset of floor plate expression. Whole-mount X-gal staining of embryos between 6 and 13 somites from reporter lines of mice expressing wild-type Rc9 (A,C,E,G), or mutated homeobox Rc12 (B,D,F,H) transgenes. Ventral midline expression from Sfpe2 is delayed as early as the 9-somite stage in embryos expressing Rc12 (compare C with D). Ectopic lacZ expression was detected in the somites of one of the lines carrying the homeobox mutation. G and H show ventral views of the floor plate after the notochord has been removed as shown in the diagram. Transverse sections of spinal cords from Rc9 (I) and Rc12 (J and K) expressing embryos stained for lacZ mRNA by whole-mount in situ hybridization. The floor plate expression of lacZ in Rc12 transgenics was patchy and thus appeared on sections as either absent (J) or weak (K). Brackets in C,D indicate extent of cranial reporter activity. Broken lines in E,F indicate the thoracic regions of the spinal cord that are magnified in G,H.

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