Expression of Cyp26C1. A. At e3.5, expression characterises the roof plate, rhombic lip and ventral hindbrain (out of focal plane). B. Transverse section through caudal rhombomere 1 showing roof plate expression (arrow). C. Transverse section through the caudal hindbrain (dashed line in A) showing expression in the roof plate, rhombic lip and in cells adjacent to the floor plate. D. Expression at e5 is similar to that at e3.5. E. Transverse section at the level of rhombomere 4 shows that ventral Cyp26C1 expression is limited to the ventricular layer. F. In addition, at the level of rhombomeres 5 and 6, Cyp26C1 characterises a discrete column of post-mitotic neurons (arrow). G. Rat e6.5, a dorsal view of the hindbrain shows Cyp26C1 is still present in the roofplate. H. Ventral view of the same hindbrain shows three ventrolateral Cyp26C1-positive populations in the caudal hindbrain. I. Transverse section through fourth ventricle roof plate (indicated in G). J, K, L. Transverse sections through the caudal hindbrain at positions indicated in H show expression coincident with elevated levels of RXRγ (Fig. 5J).