Fig. 9. (A) Illustration of STAT1 cellular localization. Following ligand binding to receptors, latent cytoplasmic STAT1 is tyrosine phosphorylated by JAKs and phosphorylation causes dimerization (1). The dimers are imported into the nucleus where they bind to a DNA target site in the promoters of responsive genes (2). The NES of STAT1 is masked when STAT1 dimers are bound to DNA. A nuclear tyrosine phosphatase can dephosphorylate STAT1, releasing it from DNA (3). The NES of STAT1 is now accessible to CRM1, resulting in export to the cytoplasm (4). (B) Alignment of STAT1 NES with putative NES sequences of other STAT family members.