Competition scan for SSP binding sites. The probe in all lanes is derived from the -50 region of the human γ promoter, a well-characterized SSP binding site.36 Binding assays were done using standard conditions and K562 extracts. Unlabeled competitor was added in the prebinding step at a level of 100 ng. Lane 1 contains no added competitor. Lane 2 represents self competition. All other lanes contain one of the FP probes (indicated above the lane) as specific competitor. The position of the Sp1 complex, SSP complex, and another Sp1-related complex (S) are indicated at left.