Publisher Summary This chapter discusses how muscle protein isoforms are expressed in the developing muscle and how the expressions of various myofibrillar proteins are interrelated. The regulatory mechanisms of protein assembly that leads to thin and thick filament formation are also presented. Myofibrils, the contractile apparatus of vertebrate striated muscles, have highly organized cross-striated structures. Both contractile and regulatory proteins having different functions are localized in the precise regions of this structure. Multiple isoforms have been distinguished in each myofibrillar protein by applying various methods including immunocytochemistry with highly specific antibodies, recombinant DNA technology, electrophoretic procedures, and protein biochemistry. Isoforms are the protein variants that share basic chemical and functional characteristics and exhibit homology in primary amino acid sequence. The expression of myofibrillar protein isoforms within individual muscles changes during the course of muscle development and regeneration, and embryonic or cultured skeletal muscle cells in which myofibrillogenesis is in progress synthesize the isoforms that are not seen in the mature muscle. Since actin is a major constituent of thin filaments, the assembly of actin is important primarily in thin filament formation. Myogenic cells derived from mesodermal cells already contain actin, even before terminal muscle differentiation. The amount increases remarkably as the muscle develops.