KIF21A and congenital fibrosis of the extraocular muscles: Whitman et al. summarized the results of previous researches and found that the variants of KIF21A, TUBB3, PHOX2A, TUBA1A, COL25A1 and TUBB2B were identified in 55%, 35%, 10%, < 1%, < 1% and < 1% of the CFEOM patients, respectively [22].