Congenital fibrosis of the extraocular muscles (CFEOM) encompasses several different inherited strabismus syndromes characterized by congenital restrictive ophthalmoplegia affecting extraocular muscles innervated by the oculomotor and/or trochlear nerves. Classic CFEOM is characterized by bilateral blepharoptosis and ophthalmoplegia with the eyes fixed in an infraducted position about 20 to 30 degrees below the horizontal midline. Involvement of the horizontal extraocular muscles is variable. If all affected members of a family have the classic phenotype with bilateral involvement, the disorder is referred to as 'CFEOM1' (Engle et al., 1997; Heidary et al., 2008). CFEOM2 (602078), an autosomal recessive disorder caused by mutation in the ARIX gene (PHOX2A; 602753) on chromosome 11q13, is characterized by bilateral ptosis with eyes fixed in an exotropic position. The CFEOM3 phenotype shows more variable clinical features: affected individuals may have unilateral eye involvement, may be able raise their eyes above midline, or may not have blepharoptosis. CFEOM3 is diagnosed in a family if even 1 member does not have classic findings of the disorder. CFEOM3 is a genetically heterogeneous disorder; CFEOM3A with or without extraocular involvement (600638) is caused by mutation in the TUBB3 gene (602661) on chromosome 16q24; CFEOM3B is caused by mutation in the KIF21A gene (608283) on chromosome 12q12; and CFEOM3C (609384) maps to chromosome 13q. CFEOM4 (609428), also known as Tukel syndrome, maps to chromosome 21q. CFEOM5 (616219) is caused by mutation in the COL25A1 gene (610004) on chromosome 4q25. See also NOMENCLATURE.