Motile cilia beat rhythmically to propel cell movement or drive extracellular fluid flow. The functional importance of cilia motility in human health is highlighted by primary ciliary dyskinesia (PCD), a genetic disease caused by cilia motility defects. Patients with PCD display left-right asymmetry defect, reduced fertility and progressive lung disease. Currently there is no specific therapy for PCD and management of symptoms has been the main approach. The dynein arms that power cilia motility comprise multiple components that are pre-assembled in the cytosol and many genes associated with PCD encode components of dynein arms. A separate group encode dynein arm assembly factors (DNAAFs), proteins that reside in the cytosol and facilitate the assembly of dynein arm subunits. Interestingly, multiple DNAAFs are localized in droplet shaped cytosolic foci. However, the precise function of these foci and the precise molecular function of most DNAAFs remain poorly understood. Based on extensive preliminary and published data, our central hypothesis is that the co-chaperone proteins Ruvbl1 and Ruvbl2 are core components of a novel membrane-less cytosolic assemblage that functions to coordinate the translation, folding and assembly of axonemal dynein arm components. In this project, we will combine zebrafish genetics, mouse genetics and cultured tracheal cells to test our central hypothesis. We propose two specific aims to achieve this goal. In the first aim, we will dissect the mechanism of Ruvbl1-Ruvbl2 foci formation. In the second aim, we will systematically define R2HAD components and dissect their biochemical and functional relationships with DNAAFs associated with PCD. Successful completion of this project will not only provide a molecular framework for dynein arm assembly and the etiology of PCD, but also lay the foundation for future investigation into the regulation, and possible intervention, of dynein arm assembly and cilia motility under diverse physiological and disease conditions.