The protein encoded by the KIF21A gene belongs to a family of plus end-directed kinesins (KIF) that are capable of movement toward opposite ends of the microtubules, thus, providing a solution to the intracellular transport of the neuronal axonal cargo.
Defects in the KIF21A protein are the cause of congenital fibrosis of extraocular muscles-1 (CFEOM1); a congenital ocular motility disorder characterized by restrictive ophthalmoplegia affecting extraocular muscles innervated by the oculomotor and/or trochlear nerves. Mutations in the same gene are also responsible for another rare form of the disease called CFEOM3.