NPAT links cyclin E–Cdk2 to the regulation of replication-dependent histone gene transcription

The localization of NPAT changes during the cell cycle. WI38 cells were arrested at G₀ by serum starvation as previously described (Tsai et al. 1993). At the time when the cells were released from the arrest, BrdU (final concentration 50 μM) was added into the culture medium. The cells were fixed, permeablized, and stained as described for Figure 2 at 6 h (G₁ phase), 19 h (S phase), 24 h (G₂), and 28 h (M phase) after release. The G₀ (serum-starved) cells were treated with BrdU for 24 h before being fixed. The BrdU staining for the cells in M phase is not shown.