For example, kynurenine (Kyn), a tryptophan metabolite in the dominant kynurenine pathway, was shown to be produced by breast, head and neck, and brain (glioblastoma) cancers at levels sufficient to activate the AHR (e.g., ~90 μM) [88,90,108] (Figure 4, black font). This evidence concerns the gene AHR and cancer.