The SERPINA1 gene encodes alpha-1-antitrypsin (AAT), also known as protease inhibitor (PI), a major plasma serine protease inhibitor. AAT helps control several types of chemical reactions by inhibiting the activity of certain enzymes. AAT complexes, predominantly, with elastase, but also with trypsin, chymotrypsin, thrombin, and bacterial proteases. The most important inhibitory action of AAT is that against neutrophil elastase (ELANE), a protease that is released from white blood cells to fight infection. AAT protects the lungs from neutrophil elastase, which can damage lung tissue if not tightly controlled by AAT. Also, AAT prevents the digestive enzyme trypsin from proteolytic cleavage of proteins until trypsin reaches the intestines. On the other hand, elevation of alpha1-AT serum levels have been observed in the course of a large number of malignant diseases.