Enforced expression of Cul4A promotes proliferation in proerythroblasts induced to differentiate. G1E-ER4 cells were induced to differentiate into erythrocytes, and Cul4A and β-globin protein levels were measured by immunoblot of 10 μg total lysate. The amounts of immunoreactive protein were quantified and normalized with respect to β-actin. DNA content was determined by propidium iodide staining and flow cytometry, and the relative number of cells in S phase was determined. Representative results from 3 independent experiments are shown. (B) Cul4A-HA and empty vector control cells were plated at 2.5 × 10⁴ cells/well in triplicate in 96-well plates, induced to differentiate, and cultured for 48 hours. Then tritiated thymidine incorporation during 8 hours was quantified. The means (cpm ± SEM) of 2 independent experiments are graphed. *P = .01. (C) Cul4A-HA (▴) and empty vector control cells (▪) were induced to differentiate, and aliquots of cells at 0, 24, and 48 hours after induction were stained with propidium iodide and analyzed by flow cytometry to determine the proportion of cells in each phase of the cell cycle. The mean (percent ± SEM) in either G₀/G₁ or S phase is graphed with respect to hours after induction for 3 independent experiments.