The Janus kinases (JAK) are a family of tyrosine kinases that play a critical role in cytokine signaling. The family consists of JAK-1, JAK-2, JAK-3 and TYK-2, and the down-stream substrates of this family of kinases include the signal transducer and activator of transcription (STAT) proteins. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses, such as allergies and asthma; autoimmune diseases; as well as in solid and hematologic malignancies[1].