Fig. 6. Loss of Tbx1 function specifically disrupts the expression of Fgf receptor 1 in the caudal pharyngeal arches. (A-C) Whole-mount in situ hybridized 35ss embryos of the indicated genotypes. Note the Tbx1 dose-dependent reduction in Fgfr1 expression in arches 3-6 (red arrowheads and bracket), and the preserved expression in arches 1 and 2, and the limb bud. This finding was consistent in 6/6 heterozygotes and mutants assayed. (D-E'') Sections reveal decreased Fgfr1 expression in unsegmented endoderm/mesenchyme of the mutant (red arrowheads) compared with the control (black arrowheads).