In 2009, the drug ecallantide, a plasma kallikrein inhibitor (KALBITOR®, investigation code DX-88, Dyax Corp/Genzyme Corp) was approved by the U.S. Food and Drug Administration as the first-in-class inhibitor for the treatment of acute attacks of hereditary angioedema (HAE), a life-threatening disorder caused by the genetic deficiency of C1 esterase inhibitor [92]. Here, C1S is linked to hereditary angioedema.