In this regard the recent reports of the ability of 5-FU to promote oxidative stress in colonic cancer cells [16,17], in addition to its multiple actions as an anti-metabolite, including inhibition of thymidylate synthase, may provide an explanation for the clear benefit of 5-FU based treatment in those patients with high GST Pi (Figure 1). The gene discussed is TYMS; the disease is colonic neoplasm.