The present invention discloses a novel protein called Synoviolin and a gene that encodes it. This protein is expressed specifically by synovial tissue and also accompanies the presence of an auto-antibody that recognizes this protein in rheumatoid arthritis (RA) patients. The protein according to the present invention and its antibody can be expected to be used as specific diagnostic markers for RA. In addition, the gene or protein according to the present invention may be used to permit the screening of drugs to treat RA. Moreover, the present invention provides synoviolin gene transgenic animals. The transgenic animals according to the present invention can be used as RA model animals in the development of pharmaceuticals to treat RA.