STAT1, also known as STAT91, is a member of the STAT protein family. In response to cytokines and growth factors, STAT family members are phosphorylated by the receptor associated kinases, and then form homo- or heterodimers that translocate to the cell nucleus where they act as transcription activators. STAT1 is activated by peptide hormones, growth factors and cytokines, particularly cytokines of the interferon family.