Fig. 2. Using I-SceI-mediated transgenesis the expression of a dominant-negative BMP receptor can be targeted effectively to lateral mesoderm during somitogenesis. Shown are representative embryos processed for in situ hybridization using an antisense RNA probe specific for the Xenopus BR transgene (br). Embryos were either injected with the I(lmo2;BR)I transgene (A-C) or represent uninjected sibling controls (D-F). Transgene expression is not detected in lateral mesoderm prior to somitogenesis, e.g. at the tailbud stage (A,D). However, at the three-somite stage (B,E) or the eight-somite stage (C,F), signal for the br transcript is detected throughout the two stripes of rostral and caudal lateral mesoderm (arrows in B and C), consistent with the normal Lmo2 expression pattern. Using this Xenopus-derived RNA probe all embryos (whether transgenic or not) showed a low level of non-specific background staining (indicated by the arrowhead in A).