Since its discovery in 2000, IL-21 has been shown to perform antitumor and antiviral functions and to participate in the development of autoimmune diseases via the Janus kinase (JAK)-signal transducer, the activator of transcription (STAT), the mitogen-activated protein kinase (MAPK) and the phosphoinositide 3-kinase (PI3K)-AKT signaling pathways [44]; however, the mechanism underlying the role in KD is unknown. The gene discussed is IL21; the disease is autoimmune disease.