Regrettably, it is important to point out that Kyn is an essential ligand for aryl hydrocarbon receptors (AHR), which will lead to activity of the Kyn-AHR axis, thereby up-regulating the gene expression of the unfavorable factors on the tumor therapy, such as TCDD-inducible poly-ADP-ribose polymerase (Tiparp) and cytochrome P450 1B1 (CYP1b1), which further expand metastasis, invasion, and immunosuppression of tumor [13, 14]. This evidence concerns the gene AHR and neoplasm.