A gene encoding a novel membrane protein polypeptide having pre-B cell growth-supporting ability, and a novel membrane protein polypeptide consisting of 180 amino acid residues having pre-B cell growth-supporting ability or a part of it. A method for producing said novel membrane protein polypeptide by preparing transformants by transforming a host cell with a vector containing said gene and culturing said transformants. A monoclonal antibody recognizing said novel membrane protein polypeptide. The above gene encodes the membrane protein having pre-B cell growth-supporting ability. The homogeneous and purified novel membrane protein polypeptide can be produced in large quantities and in addition, monoclonal antibodies recognizing said polypeptide can be produced; thus it becomes possible to identify rheumatoid arthritis (RA) and also prepare reagents for the clinical diagnosis thereof.