Fig. 4. Myf5-III fails to support correct expression in myotomes and limb buds. ß-Galactosidase staining of transgenic embryos at E9.5 (A), E11.5 (B) and E13.5 (C) reveals expression in caudal but not in rostral somites at all stages of development. Transverse section of the embryo in (A) illustrates that the somitic expression is mostly limited to the ventral dermomyotome, with very few positive myotomal cells (D). At E13.5, distinct muscles at the base of the limbs weakly express the transgene in whole-mount (arrowhead, C) and in a section (arrow, E) of the hindlimb. Myf5-III is ectopically expressed in head mesenchyme and in tailbud notochord. Expression in branchial arches and later in head muscles reflects the activity of the proximal branchial arch enhancer present in Myf5-III. The incorrect activity in somites is probably due to the enhancer located within the Myf5 gene body, as demonstrated previously (Summerbell et al., 2000).