N-acetyl cysteine (NAC) is a stable form of the non-essential amino acid cysteine. It is a sulfur-containing amino acid that acts as a stabilizer for the formation of protein structures, and promotes the formation of glutathione. Glutathione is a powerful free radical scavenging compound that also helps to maintain normal, balanced immune system function. In addition, NAC can help to support healthy brain and neuronal tissues.