MAPK8 and rheumatoid arthritis: ERK1 and ERK2 are key to the regulation of cell differentiation, proliferation, and survival.205 The main effect of JNK MAPKs in RA is cartilage destruction mediated by matrix metalloproteinase (MMP).206 Similarly, P38 is the most important member of the MAPK family linked to the inflammatory response in rheumatoid arthritis.