To determine the binding epitope of SZ-34, we prepared a series of recombinant VWF fragments, including A1A2A3, A1, A2, A3 and D'D3 ( Figure 5A ), and five GST fusion VWF-A2 fragments, i.e. A2-1, A2-2, A2-3 (VWF73), A2-12 and A2-23 (VWF114) ( Figure 5B ). We measured the binding capacities (dissociation constants, Kd) of SZ34 to native and guanidinedenatured VWF and various VWF fragments by ELISA method. SZ34 bound to native and pre-denatured full-length VWF, A1A2A3, and A2, but only native A2-12 and A2-23 (Table 2) . Furthermore, the binding of SZ34 to the pre-denatured VWF and VWF fragments was much weaker than the native counterparts ( Table 2 ). These data suggest that the epitope of SZ34 is conformationally sensitive. The binding site was mapped to the central A2 domain of VWF. Because SZ34 bound to both native A2-12 and native A2-23, the epitope of SZ34 appeared to be located within the A2-2 (A1555-G1595 region).