The alternative pathway (AP) of the complement system is crucial for innate immunity and its dysregulation may lead to several diseases, including paroxysmal nocturnal hemoglobinuria (PNH). Alnylam Pharmaceuticals Inc. and Regeneron Pharmaceuticals Inc. recently presented data on ALN-CFB, a siRNA targeting hepatic complement factor B (CFB) that inhibits the production of CFB in the liver.