Tax-inducible complex formed between THP-1 nuclear extract and 6/IL-1 oligonucleotide contains not only NF-IL6, but also other protein(s) than NF-IL6. DNA binding reactions were performed with a radiolabeled 6/IL-1. Nuclear extract prepared from Tax-expressing THP-1 cells was used in lanes 1 to 4. Lane 5 shows the mobility of NF-IL6 homodimer formed with recombinant full-length NF-IL6. Nuclear extract was incubated with anti-NF–IL6 Ab at room temperature for 30 minutes and then assayed for binding activity to the radiolabeled 6/IL-1 probe. Triangle indicates increasing anti-NF–IL6 Ab concentrations. The total amount of added antibody in each reaction mixture was kept constant by the addition of a control antibody.