TDO2 and neoplasm: The kynurenine metabolic pathway is intensified in tumor tissue, which is explained by the activating effect of the c-MYC proto-oncogene, as well as genes overexpressed in tumors and responsible for the enzymes of kynurenine production: tryptophan-2,3-dioxygenase, indoleamine-2,3-dioxygenase and arylformamidase [29].