Transformed cells designed to express a recombinant human SMBP at an elevated level to the extent that its ligand-binding activity can be measured, and cellular membrane fractions thereof; recombinant human SMBPs isolated from the transformed cells or the cellular membrane fractions thereof; a screening system for human SMBP agonists/antagonists characterized by utilizing the transformed cells, the cellular membrane fractions thereof or the isolated recombinant human SMBPs; and human SMBP agonists or antagonists obtainable by the screening system, are provided by deleting the polythymidine sequence from the base sequence of the 3'-untranslated region.