Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia and flagellar axoneme. Forms filamentous polymers in the walls of ciliary and flagellar microtubules (By similarity). Contributes to normal sperm motility (PubMed:17244819). Found in the abaxial (convex) surface of outer dense fibers in sperm flagella. Detected in testis, where it is weakly expressed in round spermatids, and strongly expressed in the flagellum of step 16 elongated spermatids (at protein level) (PubMed:17244819). In the sperm flagellum, localizes to the principal piece and midpiece (at protein level) (PubMed:17244819, PubMed:16596631). Specifically expressed in testis; not detected in other tissues tested (PubMed:17244819). Detected in testis from postnatal day 16 onwards, reaching maximal levels by postnatal day 18. In a 129S5/SvEvBrd genetic background, males show progressive reduction in fertility with almost complete loss of fertility after 5 months of breeding. Testis weight and histology appear normal. Spermatozoa have significantly reduced forward motility. The sperm flagellum shows defective bending in the midpiece region which impairs waveform propagation and forward propulsion. Sperm ATP levels deplete significantly over time, probably as a result of excess energy consumption from inefficient flagellar beating. The ultrastructure of the flagellum has some subtle abnormalities with an enlarged space between the mitochondrial sheath and the outer dense fibers. In a mixed C57BL/6J;129S5/SvEvBrd genetic background, male fertility is not significantly affected. Belongs to the tektin family.