KIF21A and congenital fibrosis of extraocular muscles: Horizontal jerk nystagmus was present in six patients with CFEOM1 carrying the c.2860 C > T(p.Arg954Trp) variant of KIF21A, and four patients with CFEOM3 had the c.904G > A(p.Ala302Thr) and c.1228G > A(p.Glu410Lys) variants of TUBB3. Upbeat nystagmus was found in a patient (S21) with the c.904G > A(p.Ala302Thr) variant of TUBB3, and multidirectional nystagmus was identified only in one patient (S16) with the c.1228G > A(p.Glu410Lys) variant of TUBB3.