Cysteine (abbreviated as Cys or C) is an α-amino acid. It is not an essential amino acid, which means that humans can synthesize it.
Cysteine is an important structural and functional component of many proteins and enzymes.
Although classified as a non-essential amino acid, in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption syndromes. Cysteine can usually be synthesized by the human body under normal physiological conditions if a sufficient quantity of methionine is available.
Cysteine is found in most high-protein foods, including: eggs, milk, whey protein, ricotta, cottage cheese, yogurt, pork, sausage meat, chicken, turkey, duck, as well in lesser amounts in vegetarian sources including red peppers, garlic, onions, broccoli, brussel sprouts, oats, granola, wheat germ.