HL-60 cells express a DNA-binding activity that specifically binds a 44-bp fragment in the p21 promoter. Whole cell extracts of log-phase HL-60 cells were incubated in the presence of a radiolabeled 44-bp fragment from the p21 promoter and subjected to electrophoretic mobility shift analysis. Binding was performed in the presence of 30× or 100× excess of specific (44) or non-specific (YY1) unlabeled competitor DNA as indicated.