The purpose of this study is to evaluate the ability of cysteine supplementation to increase glutathione production and concentrations in critically ill babies. Furthermore, the investigators want to evaluate whether cysteine supplementation results in less oxidative tissue injury and ultimately less severe illnesses. The study will enroll babies admitted to the UCLA Medical Center Neonatal Intensive Care Unit (NICU) and they will be chosen at random and in a blinded fashion to receive either cysteine or non-cysteine supplementation to their routine TPN. Small blood samples along with a single 6 hour infusion of a non-radioactive, stable isotope labeled amino acid will be used to measure the production of glutathione as well as other compounds in the blood to give a quantitative assessment to the severity of illness. Clinical information relevant to the babies' illness and subsequent recovery will be recorded.