Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia and flagellar axoneme (PubMed:15340058). Plays a key role in the assembly or attachment of the inner dynein arm to microtubules in sperm flagella and tracheal cilia (PubMed:15340058). Forms filamentous polymers in the walls of ciliary and flagellar microtubules (PubMed:15340058). May interact with CCDC172. Colocalized with CCDC172 at the perinuclear region (PubMed:24394471). Testis specific. Localized in the flagella of elongating spermatids from developmental step 15 to maturity. Tyrosine phosphorylated. Belongs to the tektin family.