Schematic diagram of krox20 expression in control embryos (A), and in embryos with alterated RA signaling (B, C). Alternation of RA gradient induces shift of the expression domain of krox20. The expression of krox20 labels rhombomeres 3 and 5 (A, yellow). Co-injection of lacZ and cyp26 or rdh10 or rdh2 mRNA into one blastomere of two-cell stage Xenopus embryos is indicated by LacZ staining (B, C, blue). The uninjected side is served as control. Ectopic cyp26 at one side of the embryos causes posterior shift of krox20 (B, yellow). In contrast to cyp26, overexpression of rdh10 and radlh2 leads to anterior shift of krox20 (C, yellow). This diagram is based on the previous reports and our unpublished data [46, 92, 129] (RKT Kam, Y Chen, WY Chan and H Zhao. Dhrs3 attenuates the retinoic acid signaling and is required for early embryonic patterning. Submitted.).