| We report the expression cloning of a novel leptin binding protein of the immunoglobulin superfamily ( OB BP 1 ) and a cross hybridizing clone ( OB BP 2 ) that is identical to a recently described sialic acid binding 1 type lectin called Siglec 5 . ^^^ Recombinant forms of the extracellular domains of OB BP 1 , OB BP2 / Siglec 5 , and CD33 / Siglec 3 were assayed for specific binding of leptin . ^^^ We also propose a role for OB BP 1 in leptin physiology , as a molecular sink to regulate leptin serum levels . . ^^^ OB BP1 / Siglec 6 . a leptin and sialic acid binding protein of the immunoglobulin superfamily . ^^^ |