Thrombotic thrombocytopenic purpura (TTP) is due to a genetic or acquired severe deficiency of the von Willebrand factor-cleaving serine protease ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), allowing unrestrained adhesion of the von Willebrand factor multimers to platelets and microthrombi formation, resulting in thrombocytopenia, hemolytic anemia, and tissue ischemia, mostly in the kidney and brain. The gene discussed is VWF; the disease is thrombotic thrombocytopenic purpura.