ID1 and ID2 are direct retinoic acid target genes in NB4 cells. Quantitative PCR and Northern blot analysis of NB4 cells treated with ATRA. mRNA was isolated from NB4 cells and ID1 (A) and ID2 (B) expression was determined using specific primers and probes by quantitative PCR (n = 4). Quantities were normalized based on β-actin expression. To investigate whether the induction of ID1 and ID2 was dependent on intermediate protein production, cells were treated with cycloheximide alone (4 μg/mL) or with the combination of cycloheximide and ATRA (10⁻⁶ M). Blots were hybridized using radiolabeled ID1-specific (C) and ID2-specific (D) probes. As a control for equal loading, blots were stripped and rehybridized with GAPDH-specific probes.