Fig. 2. A Rarg-null mutant background rescues the Cyp26a1-null mutant caudal phenotype. In comparison to a wild-type littermate (A, left), two 10-day-old A1^-/--/- double mutants (A, right) show kinked and/or shortened tails. Compared with a wild-type skeleton (B) at E18.5, an A1^-/--/- (C) skeleton reveals that the tail vertebrae have a normal morphology, although tail development is reduced by approximately four vertebral condensations. The wild-type skeleton (D) exhibits six lumbar (L1-L6; L1 not shown), three fused sacral (S1-S3) and caudal tail (tl) vertebrae. The ilium (il) articulates with S1 and the pelvic bone (pb), which articulates with the hindlimbs. While the skeleton of the A1^-/--/- mutant (E) is comparable with that of the wild-type animal, the A1^-/- (F) and A1^-/-+/- (G) mutants develop abnormally; both skeletons show deformed and abnormally fused lumbar vertebrae (L*), with the A1^-/- mutant being more severely affected. Furthermore, in the A1^-/-+/- mutant, the malformed pelvic bone (pb*) is connected to abnormally twisted hindlimbs and only six rudimentary caudal vertebrae contribute to the tail (tl*). The A1^-/- mutant exhibits a similarly deformed pelvic bone and twisted hindlimbs, as well as a more severe caudal truncation (ct), developing only three sacral rudiments.