GTPCH-expressing fibroblasts, but not the control cells, synthesized BH4 (Supplementary Table S4) and triggered phosphorylation in cocultured tumor cell lines, MDA-MB231 and BT474, on Akt (Ser473) and ERK (Tyr202/Tyr204) (Supplementary Figure S2A). Here, AKT1 is linked to neoplasm.