Fig. 2. Defective specification of first branchial arch undifferentiated core mesoderm in Pitx2 mutants. (A-C) Myogenin and (D,E) MyoD fail to be expressed normally in Pitx2 mutant embryos. (F-I). MyoR, a marker of undifferentiated core mesoderm, fails to be expressed in the Pitx2^null mutant embryos while another marker of undifferentiated progenitors. (J,K) Pitx2 expression in control and Tbx1^null mutant embryos. Arrows denote first branchial arch hybridization signal or absence of signal. Arrowheads in F,G denote signal in the second branchial arch.