Fig. 8. The limb-specific enhancer is located within the first 270 bp of the H1 element (Myf5-VI), whereas the rest (Myf5-VII) fails to support expression in limbs. lacZ staining of whole-mount embryos carrying Myf5-VI (A-D) revealed somitic expression at E9.5 (A) that is maintained until E13.5 (D). Notice, however, that expression in cervical somites is lacking at E9.5 but becomes activated at later stages (B-D). Myf5-VI is accurately activated in limb buds from E11 (B) onwards and continues in limb muscles of E12 (C) and E13.5 (D) embryos. By contrast, Myf5-VII is not expressed in limbs, and cervical somites also fail to express the transgene (E,F). Moreover, expression of Myf5-VII is not correctly maintained in most trunk somites at E13.5 (F). Both transgenes are ectopically expressed in the dermomyotome as shown on transversal sections (A',E').