Anti–c-fos and anti–c-jun antibodies inhibit DNA-protein complex formation. Gel mobility shift experiments were performed with ³²P end-labeled 75-bp enhancer fragment (bp −1578 to bp −1503) and 1 μg of nuclear extract from either uninduced K562 cells (K562) or K562 cells induced for 4 days with phorbol dibutyrate (4d Ind). Monoclonal or polyclonal antibodies against either c-fos or c-jun (at the indicated concentration) were preincubated with nuclear extracts for 18 hours at 4°C before the addition of ³²P end-labeled probe, as described in Materials and Methods. Anti–c-fos and anti–c-jun inhibited DNA-protein complex formation.