Sameem et al., 2019, in a book about nonvitamin and nonmineral nutritional supplements, illustrate that L-cysteine "is a proteingenic amino acid, which contributes toward a multitude of functions in biological systems". Classified as a "semiessential amino acid", it can be synthesized in the liver. "As a building block, L-cysteine contributes 2% of the total structural proteins in the body in the form of connective tissue, cell membranes, and myelin sheaths around neurons" ). An L-cysteine precursor is the N-acetyl-L-cysteine (NAC). "Studies have revealed that NAC can be helpful in treating many health problems like osteoporosis, acute respiratory diseases, and acetaminophen poisoning" .