The diagnosis was corrected to TTP several days after discharge from hospital, when the ADAMTS13 activity in the pre-treatment plasma sample was found to be unmeasurably low using the fluorescence resonance energy transfer (FRETS-VWF73 substrate) assay (Peptide International Inc. Louisville KY) [9]. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.