BTK and rheumatoid arthritis: Multiple transcription factors and signaling pathways are involved in the pathogenesis of RA; the most important and key pathways are MAPK (mitogen-activated protein kinase), NF-κB (nuclear factor kappa B), PI3/AKT (phosphatidylinositol 3 kinase-AKT, also known as PKB), JAK/STAT (Janus-activated kinase signal transduction and activator of transcription), Wnt/β-catenin (Wingless/Integrated), SYK/BTK (spleen tyrosine kinase)/Bruton’s tyrosine kinase), and Notch [19,20].