TDO2 and cancer: Kynurenine accumulation is also frequently found in the TME and results from an overexpression of indoleamine 2,3-dioxygenase 1 (IDO1) or tryptophan 2,3-dioxygenase 2 (TDO) in tumor and/or tumor-associated cells (e.g., MDSCs or cancer-associated fibroblasts).