GSK3 regulation of cyclin D1 expression in the absence of serum: NIH3T3 cells were cultured in the indicated medium, following an initial pulse with BrdU. After 11 hrs the cells were fixed, stained and analyzed. (A) The cyclin D1 level of each cells is plotted vs. its DNA content, but only cells labeled with BrdU, and therefore in S phase at the beginning of the treatment period are displayed. (B) Three identical experiments were performed and the average level of cyclin D1 in G2 phase cells following all treatments is presented. For this analysis the value for serum-deprived cells was set at 1.0, and the relative expression levels following other treatments is displayed.