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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 δ transgene (δbr). Embryos were either injected with the I(lmo2;δBR)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 δbr 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).