Antithrombin III (ATIII) is a nonvitamin K-dependent protease that inhibits coagulation by neutralizing the enzymatic activity of thrombin (factors IIa, IXa, Xa). Antithrombin III activity is markedly potentiated by heparin, the principal mechanism by which both heparin and low–molecular-weight heparin result in anticoagulation.

Congenital antithrombin III deficiency is an autosomal dominant disorder in which an individual inherits one copy of the SERPINC1 (also called AT3) gene on chromosome 1q25. 1, which encodes antithrombin III