L-cysteine, also known as cysteine, is a non essential amino acid in the human body. Amino acids are the constituent units of proteins, and proteins are the material foundation of life. From humans to microorganisms, everything is composed of proteins. L-cysteine is mainly used in the fields of cosmetics, medicine, and food. In cosmetics, it is used to prepare perm essence, sunscreen, hair perfume and hair tonic.