Cysteine is a semi-essential proteinogenic amino acid. The thiol side chain in cysteine often participates in enzymatic reactions, as a nucleophile. The thiol is susceptible to oxidation to give the disulfide derivative cystine, which serves an important structural role in many proteins. Cysteine is found naturally in a number of foods, including egg yolks, red peppers, garlic, onions, yogurt, grains such as wheat germ and oats, poultry and dark leafy vegetables like Brussels sprouts and broccoli (Wikipedia).