In mammalian cells, many antioxidant defence systems exist which protect cells from subsequent exposure to oxidant stresses. One antioxidant is glutathione (GSH), a tripeptide present in virtually all cells that regulates the intracellular redox state and protects cells from oxidative injury. It is metabolised via the gamma-glutamyl cycle, which is catalysed by six enzymes. In man, hereditary deficiencies have been found in five of the six enzymes. Glutathione synthetase deficiency is the most frequently recognised disorder. Defects in GSS can cause glutathione synthetase deficiency (GSSD aka 5-oxoprolinase deficiency, MIM:266130), a severe autosomal recessive disorder characterised by an increased rate of haemolysis, 5-oxoprolinuria, CNS damage and recurrent bacterial infections. In this condition, decreased levels of cellular glutathione result in overstimulation of gamma-glutamylcysteine synthesis and its subsequent conversion to 5-oxoproline. Glutathione synthetase deficiency can be classed as mild, moderate or severe (Ristoff & Larsson 2007, Aoyama & Nakaki 2013).