Effects of ATRA, IFNα, and the combination of the two compounds on the levels of IRF1 and OAS mRNAs in NB4 cells. NB4 cells (4 × 10⁵/mL) were treated for 4 days with medium alone (Medium) and medium containing ATRA (10⁻⁶ mol/L), IFNα (1,000 U/mL), or the combination of the two compounds. Total RNA was isolated and subjected (20 μg/lane) to Northern blot analysis. Filters were sequentially hybridized with cDNA probes coding for IRF1 and β-actin or OAS and β-actin. A Northern blot representative of three independent experiments is shown.