To study the role of Mst1 in autoimmune mechanisms involved in rheumatoid arthritis, we immunized Mst1−/− mice with CII emulsified in CFA, using a modification of the standard immunization procedure successfully adapted by Campbell et al. to induce CIA in mice derived from the C57BL/6 (H-2b) genetic background, including C57BL/6×129/Sv hybrids [36]. This evidence concerns the gene MST1 and rheumatoid arthritis.