Fig. 2. cyp26 and hoxb1b expression domains define an early AP boundary within the prospective neuroectoderm. Dorsal views of whole embryos at the stages indicated on top of. Genes analyzed are indicated at the left of the column or in the figure, with text color matching the in situ stain. cyp26 expression starts at the 30 to 40% epiboly stage (A) in presumptive anterior neural ectoderm (filled arrowhead) and at the blastoderm margin (open arrowhead), leaving a narrow gap (asterisk). As gastrulation proceeds, hoxb1b expression is initiated within the gap at about the 60% epiboly stage (C,G,K). From this time onwards, cyp26 and hoxb1b are expressed in a complementary manner in the neural ectoderm throughout gastrulation; a narrow domain of reduced expression is observed at the interface between the expression domains of these two genes (K-M, arrow). (N-Q) cyp26 expression (red) in relation to expression domains of other AP-specific genes (purple), raldh2 (N,O), iro1 (P) and krox20 (Q). There is a narrow gap between cyp26 and raldh2 expression domains at early gastrula (N, arrow), and this gap becomes wider by late gastrula (O, arrow). At the end of gastrulation, cyp26 and the anterior domain of iro1 expression share a common posterior boundary (P, arrow). There is a gap between cyp26 and rhombomere3 staining of krox20 (Q, arrow); thus the posterior boundary of cyp26 at this stage appears to be positioned around rhombomere 1.