cyp26 genes function redundantly to protect the anterior hindbrain from endogenous RA. cyp26a1 and cyp26c1 are responsible for setting the r3-r4 limit. (A,C,E,G) Knocking-down cyp26b1 and/or c1 has no effect on hindbrain patterning in wild-type embryos. (B) cyp26a1 mutants exhibit a mild expansion of r4. This effect is enhanced by knocking-down cyp26b1 (D), but hoxb1a expression is shifted to the presumptive cerebellum (En-expressing domains) by knocking-down cyp26c1 (F). Embryos depleted for all three cyp26 proteins exhibit a profound anterior shift of hoxb1 expression to the mid-hindbrain domain (H). Scale bar: 100 μm.