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. The gene discussed is IDO2; the disease is cancer.