Cysteine is a non-essential amino acid that contains sulfur, making it unique among the 20 standard amino acids. It plays a crucial role in protein synthesis and is important for the formation of disulfide bonds, which help stabilize the structure of proteins. Cysteine can be synthesized by the body from another amino acid called methionine. This amino acid is also a precursor to glutathione, a powerful antioxidant that protects cells from damage. Cysteine is found in various foods, including meat, fish, eggs, and dairy products, as well as in some plant sources like garlic and cruciferous vegetables.