A variety of cytokines induce the tyrosine phosphorylation of signal transducers and activators of transcription (STATs). Activation of the same STAT proteins by distinct cytokines and activation of different STAT proteins by each cytokine are thought to contribute to redundancy and pleiotropy of cytokine actions respectively. STAT3 is rapidly tyrosine phosphorylated in response to IL-6, ciliary neurotrophic factor, oncostatin M, leukemia inhibitory factor, IL-11, granulocyte colony stimulation factor and epidermal growth factor. In this report we have isolated and characterized the mouse genomic structure of STAT3. The mouse STAT3 gene consisted of 24 exons which spanned > 37 kb. The structure of the mouse STAT3 gene was almost identical to that of the human STAT2 gene, including the number and size of exons, indicating that the exon-intron organization had already been accomplished before these two genes duplicated, and then these genes evolved to respond to different ligands. By molecular linkage analysis with interspecific backcross mice the STAT3 gene mapped at 1.4 cM proximal to D11Mit59 on mouse chromosome 11. The promoter region contained potential regulatory elements such as GATA, NF-IL-6, PEBP2, Sp-1, AP-2 binding sites, cAMP response element, CAAT box and E-box. Transient expression of constructs harboring the 5' flanking region of the STAT3 gene fused to the luciferase gene showed that a 160 bp sequence upstream of the transcription start site conferred a basal and an IL-6-inducible promoter activity.