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Fig. 3. A Rarg-null mutant background rescues a subset of the Cyp26a1-/- cervical vertebral abnormalities. In wild-type skeletons (A), the first cervical vertebra (C1 or atlas) develops a thick neural arch and an anterior arch (AAA); anterior tuberculi (*) distinguish C6. Thoracic (T) vertebrae harbor ribs (see r1 on T1), the first seven of which articulate with the sternum. T2 vertebrae also develop a prominent dorsal spinous process (arrowhead). The A1-/- mutant skeleton (B) shows several posterior homeotic transformations (see Results), including C5 to C6 (C5*), C7 to T1 (C7*) and T1 to T2 (*). In addition, the A1-/- mutant exhibits fusion between the exoccipital bone (eo) and the neural arch of C1 (C1*), an abnormal AAA (AAA*) and bifidus of C2 (C2a* and C2b*). Whereas posterior homeotic transformations are evident in the A1-/-{gamma}-/- mutant (C), including C5 to C6 (C5*), C7 to T1 (C7*) and T1 to T2 (*) transformations, C1 and C2 develop normally. In the A1-/-{gamma}+/- mutant (D), C7 exhibits a posterior transformation (C7*); also, the neural arch of C1 and the exoccipital bone (eo) are fused.