Proinflammatory cytokines, such as IFN-γ, IL-6, and IL-12, ignite serial phosphorylation of JAK1/2, cytokine-receptor chains, and STAT1/3/4. Phosphorylated STATs form heterodimers or homodimers translocate to nucleus, leading to the transcriptions of Th1- or Th17-related genes. On the other hand, γC subunits of IL-2, IL-4, IL-7, IL-15, and IL-21 receptors are associated with JAK3 and STAT5 signaling. JAK1/2 selective inhibition spared the IL-2–JAK3–STAT5 signal and therefore may spare Tregs. Th, T helper.