Component of the central pair apparatus of ciliary axonemes. Plays a critical role in the function and structure of motile cilia. May play a role in endochondral bone formation, most likely because of a function in primary cilia of chondrocytes and osteoblasts (By similarity). Essential for normal spermatogenesis and male fertility (By similarity). Required for normal manchette structure, transport of proteins along the manchette microtubules and formation of the sperm head and flagellum (By similarity). Essential for sperm flagellum development and proper assembly of the respiratory motile cilia central pair apparatus, but not the brain ependymal cilia (By similarity). . This is the function of SPAG17 (sperm associated antigen 17, ENSG00000155761).