Two missense mutations in the sequences encoding the forkhead domain were shown to co-segregate with the dyl phenotype: two C-to-T transitions at coding nucleotides 277 and 293 that change codons 93 and 98 from phenylalanine to leucine and serine respectively (p.F93L, p.F98S). These mutations are in a region of the protein thought to be critical for DNA binding.