Janus kinase (JAK) is a family of intracellular, nonreceptor tyrosine kinases that play an important role in cytokine-mediated signaling. This family includes four protein kinases; JAK1, JAK2, JAK3 and Tyk2. Various studies have suggested that regulation of JAK activity should be a promising approach to combat numerous diseases such as immune deficiencies and cancers. The JAK3 (Janus Kinase 3) Assay Kit is designed to measure JAK3 activity for screening and profiling applications using Kinase-Glo® MAX as a detection reagent.