Emicizumab (Chugai Pharmaceutical Co., Ltd., Tokyo, Japan) [8,9], a bispecific antibody for FX and FIX, is useful for patients with life-threatening hemophilia, with or without inhibitors of FVIII, and can reduce the injection frequency for the treatment of hemophilia A. However, the measurement of FVIII activity and inhibitors is difficult to perform using the activated partial thromboplastin time (APTT) one-stage clotting assay in hemophilia A patients treated with emicizumab. This evidence concerns the gene F8 and hemophilia A.