Ultrastructural analyses of DYX1C1-mutant motile cilia demonstrate disruptions in outer and inner dynein arm (ODA/IDA) complexes. DYX1C1 localizes to the cytoplasm of respitatory epithelial cells and interacts with the cytoplasmic ODA/IDA assembly factor KTU (DNAAF2). We suspect that DYX1C1 functions in cytoplasmic pre-assembly of axonemal dynein complexes. Cilia with loss-of-function mutations show severe decrease in amplitude and frequency, most of them are complete immotile. Other DYX1C1 mutations result in defects of only distal axonemal ODA/IDA components. In these hypomorphic mutations, ciliary beat shows only slight reductions of beat frequency and amplitude.