Recent studies on Signal transduction stimulated by interferons have defined pathways chat link cell surface receptors to target genes in the nucleus. After interferon binding nonreceptor tyrosine kinases are activated that phosphorylate components of latent DNA-binding factors in the cytoplasm. These phosphorylated factors form multimeric complexes that translocate to the nucleus and bind to specific DNA sequences in the promoters of induced genes. A 91-kD factor (p91 or Stat91) is activated by interferons and serves as a subunit partner in the composition of diverse transcription factors.