Thrombotic thrombocytopenic purpura (TTP) is a medical emergency and a form of thrombotic microangiopathy (TMA), in which the activity of the von Willebrand factor-cleaving protease, a disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13 (ADAMTS13), is reduced. This evidence concerns the gene ADAMTS13 and thrombotic thrombocytopenic purpura.