Kyn levels are mainly regulated by IDO1 enzymatic activity in tumor cells.[18] Hit compounds with IC50 values <5 μm were analyzed for direct modulation of IDO1 activity which revealed that only 3.9 % of the compounds inhibited IDO1 activity (IC50≥0.33 μm), including several chemotypes that have not been linked to IDO1 inhibition before (Figure 2 A, Figure S3 and S4). This evidence concerns the gene IDO1 and neoplasm.