Protein-binding of a p21 promoter fragment is inversely associated with p21 message expression. Log-phase HL-60 cells were exposed to medium alone or medium containing DMSO (1.25%), ATRA (1 μmol/L), PMA (80 nmol/L), or butyrate (1 mmol/L) and harvested 20 hours later for RNA and protein extraction. Northern blot (A) confirms p21 induction. Gel-shift assay (B) of 20 μg whole cell extract incubated with 44-bp promoter fragment discloses a band upon addition of uninduced extract, which is decreased in lanes using differentiating cell extract.