The Allele is a Missense Mutation Within the Predicted Tryptophan 2,3-dioxygenase Protein Domain of .

Many loss-of-function mutations in the ( ) gene have been described. However, the causal mutation in the common allele is unknown. We sequenced different alleles ( , , and ) to identify candidate mutations. We identified a single T>A missense mutation shared among the three chromosomes, resulting in a Phe>Ile amino acid change within the predicted tryptophan 2,3-dioxygenase protein domain. This same T>A missense mutation has been independently shown to result in a phenotype by Nivard et al., 1993 and is therefore strong corroborating evidence that this mutation is causal for the phenotype in the allele.