Glutathione (GSH) is a tripeptide of glutamate, cysteine, and glycine, distinguished by an unusual γ-carboxyl peptide bond that resists standard peptidases. It is the most abundant intracellular thiol antioxidant. The reactive cysteine thiol neutralizes reactive oxygen species and, via glutathione peroxidase, reduces peroxides; the resulting oxidized dimer (GSSG) is recycled by glutathione reductase using NADPH. GSH is also central to phase-II detoxification through glutathione-S-transferase conjugation. Cellular GSH:GSSG ratio is a widely used index of redox status in research.