JAK1 and rheumatoid arthritis: However, among 4 JAK isoforms, JAK1 mainly regulated various cytokine signal pathways related to the pathophysiology of multiple inflammatory diseases (Kotenko and Pestka, 2000; Peeva et al., 2018), JAK2 was involved in the essential regulatory functions of granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Broxmeyer, 2013), JAK3 was more selectively expressed in hematopoietic cells and Tyk2 was a target for treating psoriasis and inflammatory bowel disease rather than RA (Norman, 2014).