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In February 2019 the FDA approved Sanofi’s caplacizumab for acquired thrombotic thrombocytopenic purpura (aTTP), a rare disease characterized by excessive blood clotting in small blood vessels. Caplacizumab is the first drug approved for this disease and the first to target von Willebrand factor (vWF), a key protein in the blood coagulation cascade. It is also the first domain antibody to be approved by the FDA, a belated but potentially lasting landmark event for this modality of small biologics.