Hydin deficiency causes a CP defect. Ependymal cilia (a and c) and image averages (b and d) of ependymal cilia from wild-type (+/+, a and b) and hy3/hy3 (−/−, c and d) animals. (e–h) Details of a–d, respectively, showing the CP of microtubules. (i–l) Details of the CP of wild-type (i) and mutant (k) tracheal cilia and image averages of wild-type (j) and mutant (l) tracheal cilia. Image averages are based on 5 (f), 13 (h), 10 (j), and 8 (l) images. (m and n) Diagram of the CP apparatus in wild-type (m) and mutant cilia (n). The CP projections are labeled in e, i, and m. The arrowhead in e and j indicates the diagonal link between the C1b projection and the C2 microtubule. The arrowhead in g, h, k, l, and n indicates the missing C2b projection in the CP of hydin mutants. Bars, 50 nm.