Platelet　endothelial　cell　adhesion　molecule　1　(PECAM-1)　is　a　cell　surface　receptor　widely　distributed　on　endothelium　and　hematopoietic-derived　cells,　and　maintain　the　integrity　of　the　blood　vessels.　PECAM-1　is　widely　recognized　as　an　endothelial　cell　marker.　The　homophilic　interaction　through　its　extracellular　domain　of　PECAM-1　(soluble　PECAM-1,　or　sPECAM-1)　is　important　to　its　functions.　However,　structural　details　for　such　dimerization　are　not　fully　understood.　Here　we　report　the　production　of　recombinant　sPECAM-1　in　large　quantity　by　Drosophila　expression　system　and　the　small-angle　X-ray　diffraction　(SAXS)　study.　The　recombinant　sPECAM-1　was　found　to　form　one　population　of　dimer,　but　not　oligomer,　and　was　able　to　bind　to　heparin　immobilized　on　a　chip　in　surface　plasmon　resonance　imaging　(SPRi)　binding　experiments.　The　results　of　SAXS　demonstrated　that　sPECAM-1　formed　a　symmetric　homodimer　of　Omega-shape　in　solution,　and　each　protomer　adopted　an　extended　conformation.　The　dimer　is　mediated　through　the　intermolecular　interactions　through　the　first　N-terminal　domain　(D1)　of　sPECAM-1.　This　model　provides　new　structural　information　for　its　homophilic　interaction　mechanism.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.