Competition assays using specific and nonspecific competitor oligonucleotides. In the absence or presence of competing oligonucleotides, 0.1 ng of ³²P-labeled XN3 oligonucleotide was incubated with H526 extract. Competing oligonucleotides were at 100-fold excess. The competing oligonucleotides corresponding to c-kit sequences were: Xma-Nae (−124/−83), XN (−125/−97), XN2 (−102/−82), XN3 (−96/−82). Labeled DNA:protein complexes were only formed in the presence of competitors that do not contain the −93/−84 Sp1 site. Competition using the consensus Sp1 oligonucleotide was used as a positive control, and the oligonucleotide containing a mutant Sp1 site (SC mt Sp1) was used as a negative control.