Several forms of HAE have been defined: [1] type 1 HAE (HAE-1), which is caused by C1 inhibitor (C1-INH) deficiency and characterized by low C1-INH level and function; [2] type 2 HAE (HAE-2), which results from C1-INH dysfunction and is characterized by normal or slightly higher C1-INH levels but impaired function; and [3] HAE with normal C1 inhibitor level and function, which is caused by a mutation in the F12 gene (HAE-FXII), the angiopoietin-1 gene (HAE-ANGPT1), in the plasminogen gene (HAE-PLG) and in unknown genes (HAE-UNK) [2]. The gene discussed is SERPING1; the disease is hereditary angioedema.