Most critically ill neonates receive their nutrition solely through the parenteral route during the acute duration of their illness, from a few days to weeks of post-natal life. Unfortunately, there is no parenteral amino acid formulation commercially available in the U.S. which includes cysteine as a significant nutrient. This is because cysteine is relatively unstable and cystine (i.e., cysteine dimer) is insoluble in aqueous solution. However, cysteine supplementation to TPN is possible in the form of cysteine-HCl if given within 24 hours prior to infusion. Subsequent provision of cysteine in this manner results in higher plasma cysteine concentrations in critically ill neonates. Although cysteine-HCl supplementation to TPN has been shown to be possible and safe, it is not routine practice in current neonatal care. Nevertheless, cysteine-HCl supplementation is standard practice in some major neonatal centers in the U.S. in order to augment the calcium and phosphorus solubility in TPN, which further benefits growing infants.