Fig. 3. Cdc7^–/–tg MEFs exhibit delayed S phase entry and slowed S phase progression. (A) Cell cycle distribution of Cdc7^–/–tg MEFs following serum starvation and release. The proportion of BrdU-positive cells was lower in the Cdc7^–/–tg MEFs (25%) than in the wild-type MEFs (35%). The gate settings for cells in G₀/G₁, S and G₂/M phases and percentage of cells in S phase are indicated. Consistent data were obtained from two independent experiments. (B) Incorporation of [³H]thymidine during 2 h pulses at indicated time points following release from serum starvation. The values represent average of three independent experiments. (C) FACS analysis for DNA content. The cells were sequentially arrested in G₀ and early S phase by serum starvation and aphidicolin treatment, respectively. At the time of aphidicolin block, Cdc7^–/–tg MEFs contain more G₀/G₁ cells, and at 3 and 6 h after release from the aphidicolin block, they contain less S phase cells than the wild-type (+/+) MEFs.