Effect of retinoic acid on cyclin E, cyclin A, and p21^Cip1 mRNA expression. Mitogenic stimulation and retinoic acid treatment of the B cells were performed as described in the legend to Fig 3B. mRNA from 25 × 10⁶ cells was extracted and analyzed by Northern blotting using ³²P-labeled cyclin E, cyclin A, and p21^Cip1 probes as described in Materials and Methods. As a loading control we have shown the residual rRNA levels present in the different lanes. μ, F(ab′)₂fragment of rabbit antihuman IgM; SAC, Staphylococcus aureuscrude cell suspension; RA, retinoic acid.