Posttranslational modification of p27^kip1 linked to cell-cycle entry is phosphorylation. After treatment with CIAP, only 2 isoforms of p27^kip1 were detected on 2D gel analysis (A, upper panels). Using bacterial-produced p27^kip1 as a control, it is clear that isoform 1 is unmodified p27^kip1, while isoform 2 is modified by a mechanism other than phosphorylation (data not shown). The disappearance of the other isoforms of p27^kip1 in response to CIAP treatment demonstrated that isoforms 3 to 7 are phosphorylated p27^kip1 isoforms (A, lower panels). The percentage of phosphorylated p27^kip1 at a steady-state level in the various indicated cell population was obtained as follows: signals from all isoforms/signals of isoforms 1 and 2. The bar graph shown in panel B was derived from at least 3 independent experiments. Error bars indicate standard deviation; values above error bars indicate the value of the graph bar (not the error bar).