(A,A') Analysis of the candidates from the rhombomere 4 (r4)-enriched self-organising map predicts Cyp1B1 to have r4-restricted expression. (C-H) In situ hybridisation reveals that Cyp1B1 is expressed in the ventral region of r4 at embryonic day (E)9.5 (green arrows). White dashed lines (C) represent plane of section in F, G & H. Black dashed line represent r4 boundaries (E) or r4 7 5 boundaries (I) (B,B') Analysis of embryos at E8.5 show that Cyp1B1 is switched on shortly after the induction of Hoxb1 in r4 (green arrows). Functional studies of Cyp1B1 suggest that it drives a RALDH2-independent retinoic acid signalling pathway . (I) Members of the Cyp26 gene family (for example, Cyp26B1; black arrow) modulate RA-signalling in the hindbrain to determine appropriate Hox expression (for example, see ). The normalised expression is represented graphically for each rhombomere +/- SEM.