Cilium- and flagellum-specific protein that plays a role in axonemal structure organization and motility (PubMed:24259666, PubMed:24574454). Involved in the control of flagellar beating in an asymmetric and planar waveform (PubMed:24259666, PubMed:24574454). Stabilizes outer doublet microtubules (DMTs) of the axoneme and may work as a scaffold for intratubular proteins, such as tektin and PACRG, to produce the beak structures in DMT1, 5 and 6 (PubMed:24259666, PubMed:24574454). Not essential for flagellar assembly (PubMed:24574454). Localizes predominantly along the length of all nine axonemal outer doublet microtubules (DMTs). Localizes to the inner junction (IJ) between A- and B-tubules of the DMTs. Associates to the basal bodies in a microtubule-dependent manner. Enters into the flagella through diffusion. Up-regulated during flagellar assembly. Exhibit erratic movements with cells continuously turning around the body axis. Display longer and sharp kinks flagella. Flagella beat in a uncoordinated and deformed non-planar manner. Lost beak-like strucure projections in B-tubules of the outer doublet microtubules (DMTs) 1, 5 and 6. Belongs to the CFAP20 family.