As Trp catabolism in the tumour does not necessarily lead to low Trp levels and Kyn can be further converted into downstream catabolites, measuring additional catabolites downstream of Kyn can provide important information about the efficacy of inhibition of Trp catabolism.7 Moreover, as several metabolites downstream of Kyn, such as KynA, XA and CA, are known AHR agonists,9,75,95,98 monitoring the concentration of these metabolites is relevant. The gene discussed is AHR; the disease is neoplasm.