TDO2 and cancer: The activation of the kynurenine pathway in cancer involves a variety of enzymes, which includes, but is not limited to IDO1 or IDO2, TDO, kynurenine formamidase, kynurenine‐3‐hydroxylase, kynurenine aminotransferase, kynureninase, 3‐hydroxyanthranilic acid oxygenase, quinolinate phosphoribosyltransferase (QPRT), nicotinamide mononucleotide adenylyltransferase (NMNAT) and Kynurenine 3‐monooxygenase (KMO).7, 10