IFN-α induces Stat complexes in the absence of IRS-proteins. Cells were either left untreated or treated with IFN-α as indicated. Cytoplasmic extracts were reacted with 40,000 counts per minute (cpm) of a ³²P-end–labeled SIE (A) or ISRE (B) and complexes were resolved by native gel electrophoresis and visualized by autoradiography. Specific complexes were identified by the addition of a 100-fold excess of unlabeled sis-inducible element (SIE) or ISRE to the reaction, as indicated. Composition of Stat complexes was confirmed by supershifting with the appropriate anti-Stat antibodies (data not shown).