A gene encoding a novel membrane protein polypeptide having pre-B cell growth-supporting ability and a membrane protein polypeptide consisting of 180 amino acid residues having pre-B cell growth-supporting ability. This gene encodes the membrane protein having pre-B cell growth-supporting ability. A method for producing a membrane protein having pre-B cell growth-supporting ability by transforming a host cell with a vector containing a gene encoding it and culturing the resulting transformants. A monoclonal antibody recognizing this membrane protein. The homogeneous and purified novel membrane protein pelypeptide can be produced in large quantities and used to produce monoclonal antibodies useful for identifying rheumatoid arthritis (RA) and for the preparation of reagents for the clinical diagnosis thereof.