N-acetyl-cysteine (NAC) is a dietary supplement with a wide spectrum of applications. It is used very frequently in general medicine. N-acetyl-cysteine’s mechanism of action as a precursor of the body’s own antioxidant glutathione is well-known. The role of N-acetyl-cysteine as a regulator of gene expression is less widely known. At this fundamental level NAC regulates several important processes triggered by particular physiological stimuli.