The discovery of JAK/STAT pathway goes back to 1950s. However, the details and importance of this signaling cascade have become clearer especially recently. The name of JAK is originated from the ancient double-faced Roman God, Janus, and the name of STAT, another component of the pathway, from the acronym of the proteins (Signal Transducer and Activator of Transcription), which fulfill physiological/pathological events of this pathway. This signaling does not only takes role in immune response of an organism but also in cell proliferation, differentiation, apoptosis as well as hematopoiesis. JAK1 and JAK2 were defined in 1989 by Wilks et al. In 1992 Shuai et al. sequenced cDNA of STAT1 and STAT2 proteins. STAT3, STAT4, STAT5a, STAT5b and STAT6 proteins were discovered in between 1993-1995. In the meantime, it has been demonstrated that interferons recruit the JAK/STAT pathway for its physiological events. Later on the pathological and clinical importance of the pathway has increased enormously, which boosted to the discovery JAK/STAT inhibitors (jakinibs) to be used in clinical practice.