A Disintegrin and Metalloproteinase with Thrombospondin Type 1 Motif, Member 13 (ADAMTS13) Depletion: In thrombotic thrombocytopenia (TTP), ADAMTS13 activity is <10%, which leads to decreased clearance of Von Willebrand factor (VWF) macro-aggregates with resultant platelet aggregation and thrombocytopenia. The gene discussed is VWF; the disease is thrombotic thrombocytopenic purpura.