Effect of retinoic acid on G1 CDKs. Mitogenic stimulation and retinoic acid treatment of the cells were performed as described in the legend to Fig 3B. Whole cell extracts were prepared as described in Material and Methods. (A) Equal amounts of whole cell extracts (200 μg) were immunoprecipitated with antibody to CDK2 followed by kinase assay with histone H1 as substrate as described in Material and Methods. (B) Cells were stimulated with μ (37 μg/mL) and SAC (0.005%) and harvested at 32 hours. For retinoic acid treatment of the cells, retinoic acid (1000 nmol/L) was added to the cell cultures at each indicated time before harvesting the cells at 32 hours. CDK6 was immunprecipitated from 600 μg of whole cell extracts and the immunoprecipitates were then assayed for kinase activity with GST-RB as substrate as described in Materials and Methods. + peptide, background activity was determined by blocking the CDK6 antibody with the specific antigenic peptide. μ, F(ab′)₂fragment of rabbit antihuman IgM; SAC, Staphylococcus aureuscrude cell suspension; RA, retinoic acid.