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Myosin-V is a motor protein responsible for organelle and vesicle transport in cells. Recent single-molecule experiments have shown that it is an efficient processive motor that walks along actin filaments taking steps of mean size close to 36nm. A theoretical study of myosin-V motility is presented following an approach used successfully to analyze the dynamics of conventional kinesin but also taking some account of step-size variations. Much of the present experimental data for myosin-V can be well described by a two-state chemical kinetic model with three load-dependent rates. In addition, the analysis predicts the variation of the mean velocity and of the randomness—a quantitative measure of the stochastic deviations from uniform, constant-speed motion—with ATP concentration under both resisting and assisting loads, and indicates a substep of size d₀ ≃ 13–14nm (from the ATP-binding state) that appears to …