Fig. 8. (A) Schematic representation of the myogenic phenotypes of Splotch mutant (similar phenotype is described for cMet^–/– and Gab1^–/– mice), and for Six1 and Lbx1 knockout mice and Mox2 (Bladt et al., 1995; Tremblay et al., 1998; Mankoo et al., 1999; Schafer and Braun, 1999; Brohmann et al., 2000; Gross et al., 2000; Sachs et al., 2000). Muscles not affected are in grey; green, orange, blue and purple muscles are missing (dark colours) or reduced (light colours) in Splotch, Six1^–/–, Lbx1^–/– and Mox2^–/– mice respectively. A thin layer in the most dorsal region represents superficial back muscles. At the limb level, upper muscle mass represents dorsal muscles, lower muscle mass represents ventral muscles. (B) Schematic representation of the genetic mechanisms underlying myogenesis in the different myogenic compartments and at different times (adapted from Birchmeier and Brohmann, 2000). Six1 is expressed but plays no role in somite differentiation (1) and in the migration of myogenic precursor cells (2) (red in parenthesis). Migration is controlled by Pax3, and Lbx1 is required for migration at occipital and limb level only (*). (3) Instead Six1 is required for MyoD and myogenin expression in limb buds (bold red).