The antiviral state induced by alpha interferon and gamma interferon requires transcriptionally active Stat1 protein.

Both of the latent transcription factors activated by alpha interferon or gamma interferon contain the Stat1 protein. The role of Stat1 in virus interference mediated by interferons was directly examined by using cultured cells expressing Stat1 protein and its variants. In the absence of Stat1, no antiviral state is established. Full complementation of the antiviral state requires full-length Stat1 which is phosphorylated on both tyrosine 701 and serine 727. The closely related signal transducer and activator of transcription protein Stat3 cannot substitute for the antiviral properties of Stat1.