Cul4B accumulated during mitosis of the cell cycle in neural progenitor cells (NPCs) in vitro. A. Cul4B and nestin double-positive NPCs were found in rat NPC cultures. x20 objective; scale bars, 4 μm. B. Cul4B accumulated only during mitosis of the cell cycle in rat NPCs. x60 objective; scale bars, 2 μm. An analyses of Cul4B density in NPCs during interphase versus mitosis revealed a significant difference (p = 5 × 10-10, two-tail t-Test). Y-axis, Cul4B levels in arbitrary units. C. NPCs undergoing DNA synthesis (green) did not express Cul4B. Cells undergoing DNA synthesis was identified by pulse-BrdU labeling right before fixation. x60 objective; scale bars, 2 μm. D. Mouse NPCs were analyzed after BrdU labeling. A comparison of BrdU-positive (BrdU+) versus BrdU-negative (BrdU-) cells shows that Cul4B levels were significantly higher in BrdU-negative cells than in BrdU-positive cells (p = 0.00016, two-tail t-Test). Y-axis, Cul4B levels in arbitrary units.