Intriguingly, heterozygous missense variants in KIF21A, which encodes an anterograde kinesin motor protein (Marszalek et al., 1999), were reported in 2003 to cause isolated CFEOM (Yamada et al., 2003; Cheng et al., 2014) and the phenotypic overlap between TUBB3-CFEOM and KIF21A-CFEOM suggested a shared disease mechanism underlying these two disorders. Here, TUBB3 is linked to congenital fibrosis of the extraocular muscles.