As a result, the overexpression of the tryptophan‐degrading enzymes such as indoleamine 2,3‐dioxygenase (IDO) and tryptophan 2,3‐dioxygenase (TDO) in cancer has led to extended research on the tryptophan–kynurenine pathway in cancer.7, 9. This evidence concerns the gene IDO1 and cancer.