Fig. 5. Whole-mount in situ hybridization of myogenin expression in embryos with reduced Tbx6 expression. Embryos were dissected at E10.5 from crosses of Tbx6^tm1Pa/+ Tg46/+ x Tbx6^tm1Pa/+ (A,B) and Tbx6^tm1Pa/+ x rv/rv (C,D) mice, and hybridized with a myogenin antisense riboprobe. The designation `normal' is given to embryos that were either wild type or Tbx6<sup>tm1Pa</sup>/+ with or without the Tg46 transgene (A), or rv/+ (C), which showed no differences in myogenin expression compared with wild-type embryos. (A,C) Myogenin is normally expressed in the myotome compartment of the differentiating somite and thus appears as evenly spaced stripes along the AP axis. Expression of myogenin in either the Tbx6<sup>tm1Pa</sup>/Tbx6<sup>tm1Pa</sup> Tg46/+ (B) or Tbx6<sup>tm1Pa</sup>/rv (D) embryo is disorganized compared with `normal' littermates, showing areas in which the myotomes are fused or form as small isolated islands in the intersegmental regions.