Recent work with several animal models has begun to uncover molecular and cellular mechanisms that regulate the formation of distinct types of fast and slow myofibers. As one example, studies with zebrafish mutants have shown that the zinc finger protein Prdm1 is required for formation of the first population of slow MyHC-expressing myocytes that form during development. The expression pattern of Prdm1 in lamprey somites is compatible with a similar function. In the mouse, Prdm1 is known to function in the differentiation of multiple non-myogeniccelllineages and is expressed in somites, though analyses of slow muscle formation have not been reported in Prdm1-null mice. Because it was not known if Prdm1 was required for slow muscle formation in Lycopene vertebrates other than teleost fish, we have now examined Prdm1 expression and function in differentiating skeletal muscle cells from the chicken. As in chickens and other vertebrates, zebrafish myogenesis proceeds through multiple cellular stages to produce the final complement of skeletal muscles. The first slow myofibers in the zebrafish form from adaxial cells of the somites in response to hedgehog signaling, and, in these cells, Prdm1 appears to promote the slow phenotype both by directly repressing fast muscle genes and by lifting Sox6-mediated repression of slow muscle genes. Prdm1 also is required for formation of an additional group of superficial slow myofibers, though this Dihydroberberine process is independent of Hh signaling, and, furthermore, many slow fibers form in the zebrafish independently of Prdm1. The hedeghog family proteins, particularly sonic hedgehog, regulate expression of the Gli family of zinc finger transcription factors that in turn regulate expression of the muscle regulatory factors including Myf5 and MyoD. Myogenesis in the developing chicken embryo proceeds through distinct stages in which multiple types of myoblasts and myofibers appear. The first differentiated muscle cells appear in the myotomal compartment of the rostral somites by HamburgerHamilton stage 14 on embryonic day 2 ; and these somitic myocytes begin to co-express both fast and slow isoforms of MyHC shortly after they form.