A, MDA-MB-436 cells were transfected with cyclin E-IFPN in the absence (pCDNA3.1) or presence of Fbw7. Whole cell lysates were analyzed by Western blot for cyclin E. Arrow, the M46 cyclin E isoform. Grb2 is a loading control. The activity of a cotransfected TRE-luciferase vector served as a transfection efficiency control (bar graph; raw values). B, nontumorigenic and tumor cells were treated with cycloheximide. Control cells were left untreated (0 min). Cells were fractionated and analyzed for cyclin E accumulation by Western blot. *, longer exposed film. C, densitometric values were used to plot the percent degradation and calculate the half-life (t1/2, min) for full-length (EL) and total LMW-E cyclin E.