Glutathione (GSH) is the most abundant antioxidant and is believed to maintain redox potential in tissues, cells, and individual compartments. However, GSH concentrations in some tumor cells and tissues have been reported to be as high as 1-10 mM, a concentration that is up to 10,000-fold higher than that of reactive oxygen species. Critical quantitative evaluation of glutathione's proposed functions suggests that glutathione is an amino acid checkpoint. In this role, glutathione contributes to regulating cell proliferation and apoptosis, pending amino acid availability.