CTSK and rheumatoid arthritis: Among these genes, Acp5, Ctsk, and Mmp9 were widely known to play critical roles in regulating RA pathogenesis and osteoclast function (Fig. 5b).29,30 Hydroxymethylated DNA immunoprecipitation with qPCR (hMeDIP-qPCR) analysis demonstrated that OI robustly suppressed DNA demethylation at the promoter regions of Acp5, Ctsk, and Mmp9 on the third day of osteoclast differentiation (Fig. S9d).