a,b, Slice through a denoised tomogram of a primary cilium showing numerous actin filaments in the space between the axoneme and the membrane. The repeats of actin filament half-twists are indicated by the magenta arrowheads in a and b. b, Actin filaments are also found in between microtubule singlets. c, 3D visualization of microtubule singlets and some actin filaments from the tomogram in a. d, Comparison of a subtomogram-averaged model of F-actin from the primary cilium (magenta) with a deposited structure (EMDB-6448) (left, longitudinal view; right, cross view). e, Immunofluorescence microscopy of MDCK-II cysts showing the colocalization of acetylated tubulin (green) (i) and F-actin (magenta) (ii) in primary cilia. The merged images are shown in (iii).