The proteins encoded in the most abundant mRNA class expressed in embryonic chick muscle have been identified and include the following: Myosin heavy chains (M-HC), myosin light chains (M-LC), alpha and beta actin, tropomyosin, vimentin, desmin, glyceraldehyde phosphate 3' dehydrogenase (GAP), muscle and brain creatin phosphokinase (CPK). Certain isozymic forms of these proteins are expressed only in differentiated muscle e.g., myosin heavy and light chains, alpha actin, and muscle CPK whereas all cells express the other forms of these proteins. In addition, the muscle specific set of proteins appears to be coordinately regulated in that all these genes are switched on synchronously during differentiation. Using defined, cloned ds cDNA probes for various of these genes we have isolated genomic sequences for alpha and beta actin, vimentin, myosin heavy chain and GAP. We are comparing the organization, structure, and regulation of the set of differentiation specific and the constitutive genes. At present we are focusing on alpha and beta actin and vimentin.