Three types of IFNs (types I, II and III) have been identified. Type I and type II IFNs have been widely reported. Type II IFNs only contain IFN-γ. IFN-γ is produced by natural killer (NK) cells and activated CD4+ and CD8+ T cells in response to the cytokines such as interleukin-12 (IL-12) and IL-18 [158]. IFN-γ binds to the type II IFN receptor composed of two subunits, IFNGR1 and IFNGR2. IFNGR1 and IFNGR2 induce the formation of STAT1-STAT1 homodimers. STAT1-STAT1 homodimers translocate to the nucleus and bind to the promoter of the IFN-γ-activation site (GAS) elements, to initiate the transcription of IFN-γ-regulated genes [159].