ADAMTS13 and thrombotic thrombocytopenic purpura: TTP is caused by a severe deficiency of the von Willebrand factor cleaving protease ADAMTS13 (a disintegrin metalloproteinase with thrombospondin type 1 motif 13) and exists in two forms: congenital TTP, an autosomal recessive disease due to mutations in the ADAMTS13 gene on chromosome 9, and acquired immune-mediated TTP (iTTP), an autoimmune disease caused by immunological tolerance loss in ADAMTS13.