IDO1 and neoplasm: In conclusion, we reviewed that MTCs can act on immune cells, such as Th17 cells, Treg cells, CD8+ T cells, ILC3, and DCs, by activating AhR and PXR, affecting IDO and MPO activity or epigenetic modification, affecting cytokine production and tumor immune infiltration, which can inhibit the occurrence and metastasis of colorectal, breast, gastric, and endometrial cancers.