Required for ciliary motility. Interacts with KIF9. Localizes in the cilium axoneme in a SPEF1-dependent manner. Expressed in brain and testis. Expressed in ciliated epithelial cells lining bronchi and oviduct, and in spermatocytes. At 15 dpc, expressed in developing choroid plexus. At P0, brain expression is limited to the ciliated ependymal cells. Defects in Hydin are the cause of spontaneous hydrocephalus 3, a lethal communicating hydrocephalus with perinatal onset. There is lethality in the first weeks of life because of hydrocephalus caused by abnormal ependymal ciliary motility. Cilia are unable to bend normally, ciliary beat frequency is reduced, and the cilia tend to stall. As a result, these cilia are incapable of generating fluid flow. Similar defects are observed for cilia in trachea. Mice do not exhibit randomization of left-right body asymmetry or situs inversus. It is uncertain whether Met-1 or Met-56 is the initiator. Truncated N-terminus. Truncated N-terminus. Contaminating sequence. Sequence of unknown origin in the C-terminal part.