IDO1 and neoplasm: In CRC, tryptophan (Trp) transporters (SLC7A5, SLC1A5) and key components of the kynurenine (Kyn) metabolic pathway (TDO2, IDO1, AFMID) are significantly upregulated, a molecular adaptation that enhances Trp uptake and catabolism to fuel tumor cell proliferation (196).