KIF21A and congenital fibrosis of the extraocular muscles: CFEOM1 is the most typical and common type of all forms of CFEOM; it is inherited in an autosomal dominant manner with complete penetrance, and is caused by mutations in KIF21A, resulting in dysgenesis of the superior division of the third cranial nerve and the motor neurons, leading to atrophy of its innervated superior rectus muscles and the levator palpebrae superioris.