Lee et al. [87] found that nonsynonymous single nucleotide polymorphisms of IDO1 in a small number of patients were associated with the occurrence of CD exacerbation, perianal disease, extraintestinal manifestations, and reduced serum KYN/TRP during the active phase, suggesting an association between hypofunction of IDO1 and increased inflammation in some patients with CD. The gene discussed is IDO1; the disease is Cowden disease.