HDAC9 and rheumatoid arthritis: Two previous reports using the rodent models of rheumatoid arthritis (rat adjuvant arthritis and murine autoantibody-mediated arthritis) showed that HDAC inhibitors (TSA, phenylbutyrate, or FK228) block proliferation of cultured synovial fibroblasts with accompanying up-regulation of cell cycle inhibitors (p16INK4 and p21Cip1) [31,32].