Ciliary motors locked closed by Shulin Motile cilia and flagella are vital cellular organelles with functions that include setting up the left-right body axis, clearing airways of mucus, and driving single-cell movements. Cilia beating is powered by arrays of dynein motors, the key force generators being the outer dynein arm (ODA) complexes. Using the protozoan Tetrahymena , Mali et al. identified a factor, which they name Shulin, that binds newly synthesized ODAs. Cryo–electron microscopy revealed how Shulin locks the dynein motors together by shutting off motor activity and facilitating delivery of ODAs from the cytoplasm to their final position in the cilia. Science , this issue p. 910