Downregulation of PrP mRNA in retinoic acid–induced differentiation of HL-60 cells. HL-60 cells were cultured in the presence of 1 mol/L all-trans retinoic and obtained at days 0, 1, and 3. Total RNA was isolated and reverse-transcribed to cDNA, which was then used for both prion and β-actin amplification by PCR. The PCR products were then Southern blotted with radioactively labeled PrP and β-actin probes. Blots were analyzed by PhosphorImager scanning.