JAK2 and rheumatoid arthritis: Although JAK3 is only expressed in blood, vascular smooth muscle, and endothelial cells, JAK1, JAK2, and TyK2 are all widely expressed in multi-tissue and multi-system.170,171 Studies have shown that JAK plays an important role in RA.172–174 STAT is a family of cytoplasmic proteins with both transcriptional activation and signal transduction functions.