Given the role of GM-CSF in RA, a number of clinical trials targeting GM-CSF or GM-CSFR have been carried out and shown rapid and sustained clinical responses without major safety concerns [18, 19], and inhibitors of JAK2, a major downstream signaling effector of GM-CSFR, have also shown clinical efficacy in RA [20, 21]. Here, JAK2 is linked to rheumatoid arthritis.