In the Kynurenine Pathway, enzymes crucial to tryptophan metabolism, indoleamine‐2,3‐dioxygenase 1 (IDO1), IDO2, and Trp‐2,3‐dioxygenase (TDO), trigger tumor immune resistance within the tumor microenvironment and nearby lymph nodes by depleting Trp or by activating the Aromatic Hydrocarbon Receptor (AhR) through its metabolites. The gene discussed is IDO1; the disease is neoplasm.