We examined the ability of the first downstream metabolite in each of the Trp metabolism pathways for their ability to rescue the growth of Trp-starved liver cancer cells: these are Kyn, 5HTP (a cell permeable intermediate of serotonin) and I3P, which is the first indole generated by IL4I1 (Fig. 7a). The gene discussed is IL4I1; the disease is liver cancer.