Tumoral attempts to ‘high-jack’ the KP do not end here, but involve additionally [73] a significant down-regulation of genes of enzymes of serotonin synthesis, namely TPH1 and DDC (ALAAD) in 10 and 16 cancers that express TDO2 and IDO1 respectively, thus ensuring that Trp is not used by the serotonin pathway and avoiding the anti-cancer effects of metabolites of this pathway. This evidence concerns the gene IDO1 and cancer.