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Fig. 2 The b1024 lesion is a nonsense mutation in cyp26b1 that causes skeletal defects. (A-B) Wild-type head skeleton stained with Alcian Blue and Alizarin Red dyes. (C) In b1024 mutants the ethmoid plate (brackets in A,C) and parasphenoid bone are narrow and the parachordal (arrows in A,C), basicapsular commissure (arrowheads in A,C), and lateral commissure (asterisks in A,C) cartilages of the posterior neurocranium are hypoplastic. (D) The midline symphysis of Meckel’s (red arrowheads and red insets in B,D) and ceratohyal (black arrowheads and black insets in B,D) cartilages are absent in b1024 homozygotes, fusing the left and right cartilage elements, and the basihyal cartilage (arrows in B,D) is hypoplastic. (E-F) A single base substitution in cyp26b1 (arrowhead) results in an in-frame stop codon (box). (G) b1024 causes a later truncation of cyp26b1 than previously characterized alleles, but still eliminates essential components of the cytochrome P450 enzyme domain encoded in exon 6. All images ventral view, anterior to the left. Scale bars = 50 μm