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TRANSCRIPTION of lymphokine genes in activated T cells is inhibited by the immunosuppressive agents cyclosporin A and FK506, which act by blocking the phosphatase activity of calcineurin^1–3. NFAT, a DNA-binding protein required for interleukin-2 gene transcription, is a potential target for calcineurin, cyclosporin A and FK506^4–11. NFAT contains a subunit (NFAT_p) which is present in unstimulated T cells and which forms a complex with Fos and Jun proteins in the nucleus of activated T cells^9,11. Here we report that NFAT_p is a DNA-binding phosphoprotein of relative molecular mass ∼ 120,000 and is a substrate for calcineurin in vitro. Purified NFAT_p forms DNA–protein complexes with recombinant Jun homodimers or Jun–Fos heterodimers; the DNA-binding domains of Fos and Jun are essential for the formation of the NFAT_p–Fos–Jun–DNA complex. The interaction between the lymphoid-specific factor …