The 5′ c-jun promoter fragment containing a mutation disrupting the AP-1 consensus fails to bind c-jun/AP-1. Side-by-side mobility shift reactions were performed with the same nuclear extract from Bcr-Abl.A54 cells and 5′ c-jun probes (150-bp inserts) labeled to a comparable activity. (A) On the left, the wild-type c-jun probe formed two band-shift complexes, which were cold-competed with nonradioactive self-probe, and were neutralized by c-jun antibody. (B) On the right, the gel mobility shift performed with the 150-bp 5′ c-jun probe containing a mutated AP-1 core sequence failed to bind c-jun. A faint band-shift complex was formed that was competed with nonradioactive self-probe that also lacked AP-1 consensus. This faint band-shift was not affected by the c-junantibody dilution that completely neutralized c-jun/AP-1 complexes in the companion reaction.