Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Thrombotic Thrombocytopenic Purpura ?
    ### Response:
    A lack of activity in the ADAMTS13 enzyme (a type of protein in the blood) causes thrombotic thrombocytopenic purpura (TTP). The ADAMTS13 gene controls the enzyme, which is involved in blood clotting.
                
Not having enough enzyme activity causes overactive blood clotting.
    