Time course of p21^WAF1/CIP1 mRNA induction of expression by either 1,25(OH)₂D₃alone or combined with RA in UF-1 cells. Cells were cultured for various durations (0 to 48 hours) with 10⁻⁷ mol/L 1,25(OH)₂D₃ (left) or 10⁻⁷ mol/L of both RA and 1,25(OH)₂D₃ (right). Northern blot analysis of p21^WAF1/CIP1 mRNA was performed by blotting total RNA (15 μg per lane). Equal loading of RNA in each lane was confirmed by ethidium bromide staining of the formaldehyde gel; thus, the densitometric reading for the relative levels of p21^WAF1/CIP1 transcript was compared with that of untreated cells.