Expression of Cyp26B1.A. Cyp26B1 expression at e4 in rhombomere 1 (arrow) characterises the cerebellar anlage and a complex patchwork of hindbrain domains, including a dense, lateral column of expression within rhombomeres 5 and 6 (white arrow). B. Locus coeruleus (arrow) and ventricular zone (*) in transverse section (at level indicated by black arrow in A). C. Transverse section through the caudal hindbrain (indicated by dashed line in A) showing expression in discrete patches of proliferating cells. D. Transverse section through rhombomere 5 (white arrow in A) reveals expression in both the ventricular layer and within post-mitotic neuronal populations. E. Distinct longitudinal patterns of expression are maintained at e5. F. Lateral view of e5 cerebellum and midbrain with locus coeruleus expression (arrow). G. Ventral view of mid-hindbrain domain of an e5 embryo showing expression in the region of the isthmo-optic nucleus (arrowhead). H. Expression at e7.5 assumes a defined rostrocaudal pattern incorporating the cerebellum, with the caudal limit at the level of rhombomere 6 (white arrow). I. Ventricular expression in a transverse section (indicated by dashed white line in H) within the cerebellum. J. Transverse section through caudal hindbrain (indicated by white arrow in H) showing expression in ventricular layer and pial membrane (pia, arrow).