The term “cytokine” encompasses an array of diverse soluble factors. One subset of cytokines includes more than 30 factors and comprises the-helical cytokines. These cytokines bind to a class of receptors known as type I cytokine receptors and include interleukins, colony-stimulating factors, and hormones (Bazan, 1990). Closely related are the receptors for the interferons (type II cytokine receptors). This superfamily can be further divided into subgroups based on the use of shared subunits (Taga and Kishimoto, 1995; Leonard, 1996). For instance, the common chain (c) is a subunit of the interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15 receptors. This feature of cytokine receptors provides one molecu- Figure 1. Structure of Jaks lar explanation for the redundant nature of cytokines. Among metazoan tyrosine kinases, the Jaks are structurally unique in that they contain a C-terminal catalytic domain and an adjacent The importance of cytokines in immunoregulation is pseudokinase domain. These segments constitute the first two Jak now well documented. IL-12 and interferon-(IFN) are homology (JH) domains, JH1 and JH2, respectively. Other con-important in promoting cell-mediated responses, and served segments within the Jaks have been noted and are identified IL-12 knockout mice have impaired T helper cell 1 (Th1) as JH domains, although their functions have not been ascertained. responses (Magram et al., 1996). Conversely, IL-4 and However, the N terminus has been found to be important in the