Skp2 expression is critical for regeneration after stress. (A) Analysis of Skp2 expression after 5-FU treatment in Skp2^+/+ mice. Q-RT-PCR for Skp2 was performed in Lin⁻-sorted cells at days 0 (white bar), 2, 4, 8, and 12 (black bars) after 5-FU injections. n = 6-10 in 4 independent experiments. (B) In vivo BrdU incorporation and DNA content were analyzed in Skp2^+/+ Lin⁻-sorted cells at the indicated time points after 5-FU injections. Bar graph shows the percentage of cells positive for BrdU. n = 3 in 2 independent experiments. (C) WBC counts in Skp2^+/+ and Skp2^−/− mice after 5-FU treatment at the indicated time points. n = 6 in 2 independent experiments. (D) Left, bar graph values indicated the percentage of Lin⁻ cells in G₀/G₁ phase at day 10 after 5-FU injection (n = 3 in a representative experiment). Right, histogram shows BrdU incorporation; overlaid are intensity of BrdU fluorescence in Lin⁻Skp2^−/− cells (light gray) and of BrdU fluorescence Lin⁻Skp2^+/+ cells (dark gray). *P < .05 vs Skp2^+/+. (E-F) CD45.2 Lin⁻-purified donor cells (1 × 10⁶) were transplanted into lethally irradiated CD45.1 mice. CD45.2⁺ cells were sorted and analyzed at 48, 72, and 96 hours after transplantation. (E) Fold change in Skp2 expression by quantitative RT-PCR in CD45.2 cells sorted at the indicate time points. n = 4 in 2 independent experiments. *P < .05 vs 48 hours. (F) Kinetics of BrdU incorporation in Skp2^+/+ (gray line) and Skp2^−/− (black line) mice; values in the line graph represent average percentage of cells positive for BrdU. n = 2-4. Panels A-C expressed as mean ± SEM; D-F expressed as mean ± SD.