A polyalanine tract upstream of the forkhead domain is a feature common to all of the forkhead transcription factors and variations in the length of polyalanine repeats have previously been demonstrated to underlie disease phenotypes, e.g., FOXL2 in blepharophimosis-ptosis-epicanthus inversus syndrome (BPES) [52] and FOXE1 in thyroid dysgenesis [53]. Here, FOXE1 is linked to thyroid gland disorder.