To be specific, patients harboring KIF21A mutations were classified as CFEOM-1, patients harboring PHOX2A mutations were classified as CFEOM-2, patients harboring mutations in tubulin-encoding genes (TUBB3, TUBB2B, or TUBA1A) were classified as CFEOM-3, patients harboring mutations in the locus of chromosome 21qter (with simultaneous oligodactyly) were classified as CFEOM-4, patients harboring mutations in COL25A1 were classified as CFEOM-5, and patients harboring mutations in ECEL1 were classified as CFEOM-6. Here, KIF21A is linked to congenital fibrosis of the extraocular muscles.