Two major entities with distinct pathophysiology have been recognized: thrombotic thrombocytopenic purpura (TTP) caused by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) and atypical hemolytic uremic syndrome (aHUS) caused by complement dysregulation. The gene discussed is ADAMTS13; the disease is atypical hemolytic-uremic syndrome.