Component of the central pair apparatus of ciliary axonemes. Plays a critical role in the function and structure of motile cilia (PubMed:23418344, PubMed:15827353). May play a role in endochondral bone formation, most likely because of a function in primary cilia of chondrocytes and osteoblasts (PubMed:26017218). Essential for normal spermatogenesis and male fertility (PubMed:29690537). Required for normal manchette structure, transport of proteins along the manchette microtubules and formation of the sperm head and flagellum (PubMed:29690537). Essential for sperm flagellum development and proper assembly of the respiratory motile cilia central pair apparatus, but not the brain ependymal cilia (PubMed:32988999). 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. Localizes to the manchette in elongating spermatids (PubMed:29690537). Highly expressed in testis, round spermatids, testicular sperm, epididymal sperm and in condensing spermatids (at protein level) (PubMed:15827353, PubMed:29690537, PubMed:32988999). Expressed in organs that contain cilia-bearing cells including brain, oviduct, lung, and uterus (PubMed:15827353, PubMed:32988999). Expressed in articular cartilage and bone (PubMed:26017218). Expressed from day 16 when pachytene spermatocytes are present. Defects in Spag17 are the cause of primary ciliary dyskinesia only (Pcdo) phenotype. Mice are generally viable to adulthood but have a significantly shortened lifespan, with chronic morbidity. Show neonatal progressive hydrocephalus, accumulation of mucus in the respiratory passages and male infertility. Deficient mice die within 12 hours of birth with severe defects in motile cilia (PubMed:23418344). Mice also have skeletal malformations (PubMed:26017218). Male mice are infertile because of a severe defect in spermatogenesis (PubMed:29690537). Spermatids display abnormally long manchette structures and defects in the morphology of the sperm head, acrosome, and tail (PubMed:29690537). Additionally, the transport of proteins along the manchette microtubules is disrupted in the elongating spermatids (PubMed:29690537). Missing sequence in a highly repetitive region between exons 21 and 22. Truncated N-terminus.