Oxidized glutathione (GSSG) is the oxidized form of glutathione (GSH), also known as glutathione disulfide, formed by the oxidation of two GSH molecules. GSSG can be reduced back to GSH by glutathione reductase; thus, the majority of cellular glutathione exists in the reduced form. Measurement of intracellular GSH and GSSG levels, as well as the GSH/GSSG ratio, provides a reliable assessment of cellular redox status.