MAPK8 and rheumatoid arthritis: Recent studies have validated the crucial roles of osteoprotegerin (OPG)/receptor activator of nuclear factor-κB ligand (RANKL), mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signal pathways in RA.