Cytokines play a pivotal role in regulating a variety of immune responses mainly by downstream gene activation or repression. To transduce extracellular signals into intracellular responses, many cytokines utilize a common pathway - the Janus kinase (Jak)-signal transducer and activator of transcription (STAT) signaling pathway. Jak-STAT signaling pathway was first discovered through the study of gene activation in response to type I and II interferons (IFNs).