Retinoic acid induces a transient increase in the association of p21^Cip1 with CDK2 complexes. Mitogenic stimulation and retinoic acid treatment of the B cells were performed as described in the legend to Fig 3B. Whole cell extracts were immunoprecipitated with antibody against CDK2. The immunoprecipitates were then examined for CDK2 (top) and p21^Cip1 (bottom) levels by Western blot analysis. IP, immunoprecipitating antibody; W, Western blot antibody; μ, F(ab′)₂ fragment of rabbit antihuman IgM; SAC, Staphylococcus aureus crude cell suspension; RA, retinoic acid.