Silencing SLC1A5 or SLC7A5 in the colon cancer cell line HCT15 (Fig. 5G) led to a reduction of 13C-Trp and 13C-Kyn observed 1 h after addition of 13C-Trp to the culture medium (Fig. 5H). The gene discussed is SLC7A5; the disease is colonic neoplasm.