The use of synthetic amino acids in swine nursery and growing diets is a common practice implemented throughout the world. The majority of the research to date has focused on the use of synthetic lysine. With the present knowledge of lysine requirements, it is becoming important to know the requirements for the next limiting amino acid (methionine and cystine, the two sulphur amino acids). Past research on the sulphur amino acid requirements indicates that optimum performance was between 8.7 and 11.9 mg/g of average daily gain. Recent research has concluded that gain and feed efficiency are maximized with a Synthetic Amino Acid: Lysine ratio between 60 and 63%.