The first two crystalline structures of the myosin head were recently published. The myosin head is made of two main domains. The motor domain comprises the first 600 N-terminal residues of the myosin head heavy chain; it contains the ATP-binding site and the main interface with actin . The transmission domain is composed of the long helicoïdal C-terminal part of the myosin head heavy chain, along with essential and regulatory light chains. New motility assays demonstrated the movement and force produced by a single myosin molecule along the actin filament. Genetics and protein engineering led to the production of chimeric myosin molecules. These combined techniques enabled to demonstrate that the motor domain of the myosin head in skeletal muscles is self-sufficient for hydrolyzing ATP and inducing movement, and that the presence of light chains is crucial for motility efficiency by stabilizing the transmission domain . In contrast, Iight chains in non-striated muscle or non-muscle myosins determine motor activity by regulating ATP hydrolysis but do not markedly influence motile performances of the corresponding muscles. Such studies on the myosin structure and structure-function relationships between heavy and Iight myosin subunits helped to redefine the specific role of certain myosin domains. All these advances have generated renewed interest in the field of biological motor molecules and myosin isoforms. The discovery of point mutations in the ventricular myosin heavy chains from patients with familial hypertrophic cardiomyopathy has accelerated new research projects in this promising field !