This is corroborated by the inability of CD3+ T cells to express IDO1, TDO or KYNU, as a result of which these cells cannot produce or degrade Kyn by currently described pathways [28,29].We deliberately chose a single-cell analysis of blood cells, since this allows the clear isolation of subpopulations using FACS and the specific stimulation with cytokines, which are also involved in the course of Crohn’s disease. The gene discussed is TDO2; the disease is Crohn disease.