hevertebrate sarcomere is a complexstructure composedof numerousproteins arranged in an exquisitely precise manner. Sarcomeric proteins are organized into interdigitating thick or thin filaments and the sliding of these filaments relative to one another constitutes muscle contraction at the sarcomere level. Consequently, an understanding of sarcomeric structure and function requires a thorough knowledge of the individual components of the thick and thin filaments, as well as their associations. Thick filaments are comprised of myosin, which provides the force required to drive muscle contraction and also plays a major structural role in thick filament formation. In addition, a family of thick filament-associated proteins plays a role in organization of the thick filament. Wehave used both molecular genetic and cell biological approaches to define the diversity of the myosin heavy chain gene family and to analyze the assembly of myosin and it's associated proteins into thick filaments.