Fig. 9. Loss of nuclear YY1 in response to inhibition of DNA synthesis, and recovery of nuclear YY1 with cell-cycle checkpoint inhibitors. Asynchronously growing CHO cells were fixed, immunostained and imaged with a confocal microscope. Images of immunodetectable YY1 (left column) and BrdU (middle column) are shown separately in gray scale and together as two-color overlay (red for YY1 and green for BrdU). Each row shows a field of cells from each of the four different treatments displayed at the same scale. (A-C) Untreated control cells. A subset of these cells are positive for both nuclear YY1 and BrdU staining (white arrows). (D-F) Aphidicolin-treated cells show loss of nuclear staining for both YY1 and BrdU. (G-I) Addition of the checkpoint inhibitor 2-aminopurine (2-AP) to aphidicolin-inhibited cells resulted in restoration of BrdU labeling and YY1 nuclear accumulation (white arrows). A few cells that did not overcome the block also failed to show YY1 (white arrowhead). (J-L) Addition of the checkpoint inhibitor caffeine (CAF) to aphidicolin-blocked cells had the same effect as 2-AP addition. Scale bars: 10 µm.