Fig. 3. Specific mutations in the -58/-56 kb distal Myf5 enhancer disturb expression patterns in somites. Mouse embryos carrying transgenes with mutations in the putative Mef3 (A,B) and Smad (C,D) binding sites show abnormal expression in somites but not in limb buds. Myf5-IV transgenic control embryos at corresponding stages are shown for comparison (E,F). Both mutated enhancers cause a shift of the expression domain from anterior to posterior halves of somites (A,C). Sagittal (A') and transverse sections (B',C',D') of E10.5 embryos (levels of sections indicated in A and C by lines) illustrate that both transgenes are predominantly expressed in the dermomyotome. The Smad mutation also leads to massive ectopic transgene activation in dispersed and presumably migrating cells at limb level (C',D') and in addition may cause a slight delay of activation in myogenic cells in limb buds (compare D with F). Note, however, that both mutations do not prevent expression in myogenic progenitor cells in limbs.