Among the 64 patients with ai‐TTP from March 2006 to April 2008, 45 patients (70.3%) had severely reduced ADAMTS13 activity to <0.5% of that of healthy patients, 15 patients (23.4%) had ADAMTS13 activity between 0.5% and 25%, and four patients (6.3%) had ADAMTS13 activity >25%. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.