From the Manufacturer:
Cysteine is a sulfur-containing non-essential amino acid that is closely related to cystine. Cystine actually contains two cysteine molecules that are joined together. Sufficient amounts of Cysteine are formed from L-Methionine (an essential amino acid) and L-Serine via a transsulfuration reaction in the body.
However, for preterm infants Cysteine is considered to be a conditionally essential amino acid. L-Cysteine serves as a precursor for the synthesis of proteins, glutathionine, taurine, coenzyme A, biotin and inorganic sulfate, and also serves as a major precursor for the synthesis of the potent antioxidant glutathione.
Without adequate amounts of glutathione in the liver, the tissues of the body can be exposed to significant oxidative stress resulting in widespread cellular damage.