signals such as TGFb, Wnt, and Hh family members (Bate, 1993; Lawrence et al., 1995; Wu et al., 1995; Azpiazu et al., 1996; Bate and Baylies, 1996; Tajbakhsh and Mary K. Baylies,* Michael Bate,† and Mar Ruiz Gomez† *Molecular Biology Program Memorial Sloan-Kettering Cancer Center Cossu, 1997). Myogenic lineages arise from these segmental units, and genes coding for components of the Sloan Kette...