Spi-1 is a predominant protein that binds to the overlapping NF-IL6/Spi–1 binding site. The −59 to +12 DT fragment was used as a radiolabeled probe. Nuclear extracts were derived from THP-1 cells untreated (lane 1) or treated with 100 ng/mL of LPS for 45 minutes (lanes 2 to 5). In lane 6, in vitro expressed recombinant Spi-1 was used. Cold competitor DNAs were described in the text and in the legend to Fig 2A. All competitors were used in a 50-fold molar excess over the radiolabeled probe. The arrow locates the mobility of Spi-1.