Although cysteine is a non-essential amino acid made from methionine via cystathionine in children and adults, most premature and term neonates have a decreased capacity to synthesize cysteine due to their low expression of the rate limiting enzyme cystathionase. As a result, plasma cysteine and cystine concentrations in critically ill neonates are low compared to healthy infants. Unfortunately, plasma cysteine concentrations of neonates receiving cysteine-free TPN remain low despite generous methionine intakes and relatively high plasma methionine concentrations in TPN fed neonates. This is particularly relevant since cysteine is the limiting substrate for the synthesis of the principal intracellular antioxidant in the body - glutathione.