Formation of a DNA-protein complex with CPRE of the mouse promoter. Five-microgram aliquots of nuclear extracts from MEL or NIH3T3 cells were incubated with the end-labeled oligomers corresponding to CPRE in the absence (−) or presence of a 125-fold (MEL) or 175-fold (NIH3T3) molar excess of the indicated oligonucleotides. The specific DNA-protein complex band is shown by the arrow.