STAT1 is associated with type I and II interferon signaling. Phosphorylation of STAT1 at Y701 leads to dimerization and translocation to the nucleus to activate gene transcription. Human STAT1 shows 93% and 94% aa identity with mouse and rat STAT1, respectively, over the region used as an immunogen. This region is identical between isoforms STAT1a (91 kDa) and STAT1b (84 kDa).