STAT4 (signal transducer and activator of transcription 4) is a member of the STAT family that is phosphorylated by receptor-associated protein kinases. STATs form homo- or heterodimers and translocate to the cell nucleus where they act as transcription activators. T helper (Th) cells differentiation is regulated by STAT4. STAT4 controls T1/T2 balance during the response to live influenza virus and isolated viral proteins. The differentiation of gamma interferon (IFN-γ)-producing Th1 and Tc1 cells after inoculation of live virus occurred independently of STAT4 expression. STAT4 has been shown to control pathogenesis of chronic obstructive pulmonary disease.