Blood coagulation factors IX/X-binding protein (IX/X-bp) is an anticoagulant isolated from the venom of the habu snake. IX/X-bp is a disulfide-linked heterodimer consisting of two homologous subunits (129 residues of subunit A and 123 residues of subunit B). The amino acid sequence of each subunit is homologous to the carbohydrate-recognition domain of C-type lectin (C-type CRD), although IX/X-bp has no lectin activity. Crystal structure of IX/X-bp shows that a central loop projects into the adjoining subunit and contributes to an intertwinned dinrer in a manner similar to domain swapping. The exchange of the central loop complete the C-type CRD fold, and forces a large conformational change on the hinge region classically concerned in Ca²⁺ and carbohydrate-binding. This may result in the disruption ofthe lectin active site, and indeed in this region Ca²⁺ is not bound. On the other hand, a concave surface created by the dimerization provides a potential binding site for factors IX and X . The domain swapping seen here provides a new function which is not expected from the original C-type CRD.