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). Interacts (via the C-terminus) with SPAG6; the interaction probably occurs on polymerized microtubules. Detected in the cytoplasm of round spermatids and in condensing spermatids. Localized to the central pair of the sperm flagellar axoneme. Colocalizes with SPAG6 on microtubules (By similarity). Localizes to the manchette in elongating spermatids (By similarity). Highly expressed in testis. Expressed in organs that contain cilia-bearing cells including brain, oviduct, lung, and uterus. The disease may be caused by variants affecting the gene represented in this entry.