Core promoter-nuclear protein complex formation is inhibited by anti-Sp1 but not anti-AP2 or anti–Egr-1 MoAbs or oligonucleotides. Gel mobility shift experiments were performed with ³²P-end–labeled 62-bp core promoter fragment (bp −30 to −92) and 1 μg of nuclear extract from uninduced and phorbol ester-induced K562 cells. MoAbs against AP2 (A) or Egr-1 (B) (at the indicated concentration) were preincubated with nuclear extract for 18 hours at 4°C before the addition of ³²P-labeled probe. Unlabeled oligonucleotides containing the Egr or Sp1 consensus sequence (at the indicated molar excess) were incubated with nuclear extracts before the addition of the ³²P-labeled 62-bp probe (C).