In the developing chicken, multiple types of somitic, embryonic, and fetal myoblasts arise and form different fast and slow types of myofibers. The first chicken skeletal muscle cells form in the somitic myotome. Mature myotomal fibers in the chicken embryo uniformly co-express the embryonic fast MyHC along with all three of the slow MyHCs, whereas the neonatal and adult fast MyHCs are not expressed in myotomal fibers. The adaxial somites of zebrafish also initially coexpress fast and slow MyHCs. Of the MyHC isoforms expressed in chick myotomal fibers, only slow MyHC2 requires functional innervation for expression. In our studies of chick embyro somite explants, we found that all myocytes reacted with both mAb F59 and mAb S58. This staining Enoxacin hydrate pattern is consistent with the pattern of fast and slow MyHC coexpression found in vivo. In addition, we found that all somitederived myocytes expressed Prdm1 protein in vitro. In contrast to a previous whole mount in situ hybridization study that did not detect Prdm1 mRNA expression in chick somites, which may have been due to lack of probe penetration, we found that Prdm1 mRNA was expressed in somite tissues and that Prdm1 protein was expressed in the somitic myotome in vivo. Furthermore, we found that expression of both fast and slow MyHC isoforms was inhibited by antisense knockdown of Prdm1 in somite cultures, suggesting that Prdm1 was required for MyHC expression in somite-derived muscle cells. Because many of the conclusions of this work rest on the specificity of the antibodies for chicken Prdm1, we have Daucosterol presented multiple lines of evidence that support the specificity of the Prdm1 staining, including: antibodies that were made to two different epitopes that are conserved in chicken Prdm1 showed similar staining in chicken cells; in contrast, an antibody that was made against a human Prdm1 epitope that is not conserved in chicken Prdm1 failed to stain chicken cells; on immunoblots, the antibodies recognized a single band of,100 kDa, which is the predicted size of chicken Prdm1;