TRAF6 and rheumatoid arthritis: We observed that in the existence of Mg132 which is the widely used proteasome inhibitor, the wild-type polyubiquitination and K48-linked polyubiquitination levels of TRAF6 were markedly decreased in USP5 overexpressed RA-FLS cells, but the K63-linked polyubiquitination of TRAF6 was not affect by USP5 overexpression (Figure 6(e)).