Increased posttranslational modification of p27^kip1 correlates with cell-cycle entry. FACS analysis shows the percentage of cells arrested in G₁/G₀ (A, left panel) in primary B cells with or without treatment with PMA for 72 hours (labeled as primary B or primary B + PMA, respectively), Burkitt lymphoma cell line BL40 (labeled as BL40), (C) in H1299 cells with or without the inducible expression of p27^kip1 (labeled as H1299, H1299 + p27^kip1) and in Cal51 cells. Right-hand panels of A and C are Western blots showing the expression levels of p27^kip1 in the corresponding cell populations used to carry out the FACS and 2D gel analysis. The isoforms 1 to 7 are indicated with arrows in panels B and D and their isoelectric points are 6.76, 6.59, 6.37, 6.12, 6.05, 5.93, and 5.83, respectively. Horizontal bars in panels A and C represent the marker used to calculate mean values of G₀/G₁ peak cell populations. PCNA indicates proliferating cell nuclear antigen.