It has been shown that the indoleamine 2,3-dioxygenase (IDO) enzyme, involved in the catabolic pathway of tryptophan (precursor of 5-HT), is overexpressed both in tumor cell and antigen-presenting cell in tumor draying lymph node, where it promotes the establishment of peripheral immune tolerance to tumor antigens [224]. The gene discussed is IDO1; the disease is neoplasm.