Fig. 5. Mesp2::Dll1^dn transgene expression and phenotype. Schematic representation of transgenic constructs (A) directing heterologous gene expression into the anterior region of the psm corresponding approximately to somitomere S-1 as indicated by whole-mount in situ hybridisation of a day 9.5 Mesp2::lacZ transgenic embryo (B). Expression of Dll1^dn detected by RT-PCR in transgenic lines #4 and #5 (C). Hes5 expression (asterisks) in tailbuds of day 9.5 (dorsal views) and 11.5 (lateral views) wild-type (left) and transgenic embryos (right) (D). Note the reduction of Hes5 in the psm of transgenic embryos. Skeletal preparation of a homozygous Mesp2::Dll1^dn line 4 mouse showing 14 ribs (E), with 8 ribs attached to the sternum (F).