Fig. 4. Mutation of the Xvent (homeo/paired) site in the -58/-56 kb distal Myf5 enhancer abolishes expression of the transgene in limb muscle progenitor cells, whereas the mutated Tcf/Lef site shows little change of transgene expression. Embryos containing the transgene that carries the mutation in the homeobox consensus sequence (Xvent) show no expression in limbs at E10.5 (A) or E13.5 (B). It also appears that expression in somites at E10.5 and particularly in trunk muscles at E13.5 is reduced with this mutant enhancer. Transgenic embryos containing the Tcf/Lef mutation (C,D) exhibit a fairly normal expression pattern in somites and muscle precursors of limbs (D). The mutation may possibly cause a minor reduction of transgene expression in somites (C) and trunk muscles at later stages (D). Transgenic control embryos (Myf5-IV) at similar developmental stages are shown for comparison (E,F).