Boundaries of the 21PBA target sequence. 21PBA binds the 3′-half of the 44-bp promoter fragment. HL-60 cell extract was incubated with either the 44-bp fragment or 23- or 29-bp fragments bearing identical sequence to the 3′ portion of the longer fragment. All targets generate the same gel-shift band, which can be specifically competed, as shown. The 44-bp fragment is significantly more effective than the shorter fragments as a cold competitor against labeled 44- or 28-bp fragments. The bottom portion of the figure depicts the positions of recognition sites for several transcription factors relative to fragment boundaries.