Stage-specific presence of p21-promoter binding activity in differentiating CD34⁺ cells. Umbilical cord CD34⁺ cells prepared with magnetic beads were expanded in differentiating medium (KL, IL-3, and G-CSF) and extracts prepared for gel-shift in parallel with p21 mRNA determination as shown in Fig 1. The faint and slightly faster band at day 3 is reproduceable. Lane 2 indicates competition by cold fragment of the band induced by HL-60 extract.