Fig. 7. A model for hindbrain patterning through regulated RA inactivation by the Cyp26 enzymes. (A) RA-responsive gene expression in Cyp26-depleted embryos. Embryos depleted of all three cyp26 genes experience unpatterned RA signaling; as a result, the three RA-responsive genes examined in this study - hoxb1a (green), vhnf1 (yellow) and hoxd4 (orange) - are expressed throughout the transformed hindbrain. (B) A `gradient free' model for hindbrain patterning through regulated RA inactivation. Dynamic patterns of Cyp26 expression in the hindbrain (blue bars) antagonize RA-dependent gene expression by eliminating RA (red bars) first in the anterior hindbrain (6-9 hpf), then in r2-r4 (9-11 hpf), and then in r2-r6 (11-12 hpf). At each point, sequential RA-responsive genes (colored bars) are limited to progressively more posterior rhombomeres. At the same time, Cyp26a1-dependent RA degradation in the trunk mesoderm suppresses global RA levels (black hammers on right side). tel: telencephalon; di; diencephalon; mb: midbrain; c: cerebellum.