Cysteine (Cys), homocysteine (Hcy),and glutathione (GSH) are the most abundant small molecule biological thiols. Owing to their characteristic redox properties and nucleophilicity, they play major unique roles in human health and disease. Because they maintain redox homeostasis through the equilibria between their reduced free thiol and oxidized disulfide forms, monitoring the relative levels of reduced and oxidized thiols in cells became a traditional means of assessing oxidative stress. Since oxidative stress is a major aspect of the development of many chronic and degenerative illnesses including cancer, autoimmune disorders,aging, cataracts,rheumatoid arthritis, and cardiovascular and neurodegenerative diseases, monitoring the levels of biological thiols has asignificant role in biomedical research and diagnostics.[1]