Fig. 2. Expression patterns of transgenic mouse embryos containing constructs Myf5-I (A-C) and Myf5-II (D-F). Myf5-I drives strong expression in the dorsal domain of somites at E9.5 (A), which extends more ventrally during later stages of developmental (B,C). Notice the expression in the hypaxial domain at the thoracic level, whereas the intermediate myotome appears to be free of lacZ staining (arrow in C). Myf5-I is also strongly expressed in limb muscles of E12.5 and E13.5 embryos (B,C), in the mandibular and hyoid arches (A), and later in hypoglossal cord (arrowhead in B). Ectopic expression is seen in head mesenchyme and in the notochord of the tail. The Myf5-II transgene is active in somites at E9.5, except for the cervical region (D), but somitic expression is not properly maintained at later stages (E,F). Also notice that ß-galactosidase staining is inappropriately confined to the posterior margin of somites (D). Myf5-II is never activated in limb buds.