Developing animals must distinguish left from right. Defects in this process lead to flipped organs in model systems and in humans and can compromise organismal physiology and fitness. In this interdisciplinary project, we will combine protein engineering with physical modeling and experiments in the fruit fly to explore how left-right asymmetry can scale from molecules to organisms. We will modify the structures of myosin proteins — related to the molecules that drive muscle contraction — in order to generate biased left-handed or right-handed motion at the molecular scale. The functions of these proteins will first be measured outside of cells, and then the proteins will be introduced into flies to test their effects on the asymmetric development of the fly gut. Light-controlled versions of the engineered proteins will enable exquisite control of chirality during development. The work will provide key tests of proposals for how left-right asymmetry scales up from molecules to cells to organs, and generate physical models of a fundamental process in biology.