Antitrypsin (also referred to as alpha-1-antitrypsin or alpha-1-protease inhibitor) is a glycoprotein produced in the liver which is the major anti-protease in the blood, serving mainly to inhibit the action of protein-destroying enzymes such as trypsin and leukocyte elastase. It is a single-chain molecule, has 394 amino acids and a molecular weight of 51,000.

Hereditary antitrypsin deficiency is one of the most common genetic disorders among males of European descent. Alpha-1-antitrypsin deficiency results in leukocyte elastase building up and breaking down the lining of the lungs, which results in a severe form of emphysema (oxidizing agents in cigarette smoke are also known to inactivate antitrypsin, thus causing the high rates of emphysema among long-term smokers).

Because over 20,000 people are affected by this deficiency in the U.S. alone, large amounts of this anti-protease are needed. To meet the demand, researchers are genetically engineering sheep that produce the antiprotease in their milk.

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