Fetal myoblasts form large Echinocystic-acid multinucleate myotubes that initially express only fast MyHC, though, after 7�C10 days of differentiation, a percentage of the fetal myotubes begin to co-express slow MyHC1 along with the fast MyHC. We examined fetal cultures within 4�C5 days of differentiation at which time only fast MyHCs were expressed in the myotubes. Though it was unexpected to find Prdm1 Cyanoacetohydrazide expression in the fetal myoblasts and myotubes, our combination of mRNA, immunoblotting, and immunocytochemical studies provided confidence that Prdm1 was expressed in the fetal limb-derived myogenic cells. In fetal cultures, as in embryonic cultures, therefore, Prdm1 expression was not limited to cells that expressed slow MyHC, but was also found in myotubes that expressed only fast MyHC. Our results suggest that the developmental pattern of Prdm1 expression differs between chickens and zebrafish. In the somites of zebrafish, for example, Prdm1 expression is limited to the adaxial somite cells and is not found in the fast myofibers. Lamprey somites have a similar adaxial pattern of Prdm1 expression. In the chick embryo, in contrast, we found that Prdm1 was expressed both throughout the mature somitic myotome and in all somitic myocytes that formed in culture. In addition, expression of zebrafish Prdm1 is limited to a subset of all of the myofibers that express slow MyHC, whereas we found that chicken Prdm1, at least in cultures, was expressed in all myotubes, including those that did not express slow MyHC such as the embryonic and fetal fast myotubes. In the mouse, the role of Prdm1 in myofiber formation and diversification has not been analyzed in detail. Similar to what we found in the embryonic chicken, Prdm1 is expressed throughout the somitic myotomes of the mouse. In somites of E10.5 Prdm1-null mouse embryos, the domain of Fgf8 gene expression is expanded, but the possible effects of Prdm1 inactivation on myotomal structure and MyHC expression in mouse somites have not been reported.Additional studies have shown that, in the mouse, Prdm1 is required for formation of posterior limb structures including musculature, though the effect of Prdm1 inactivation on limb myofiber diversification was also not specifically examined.