**Figure 2.** **Knockdown of IGFBP5 induces premature senescence in young MEFs.** (**A**) Levels of *Igfbp5* mRNA in P2 MEFs 48 h after transfection with control siRNA and siRNA against *Igfbp5*. N=5 in each treatment. ***P<0.001 by paired Student’s t-test. (**B**) Representative immunoblot for IGFBP5 in P2 MEFs transfected with control siRNA and siRNA against *Igfbp5*. kDa: kilodalton. (**C**) Representative images of SA-β-GAL staining in cells transfected with control siRNA or *Igfbp5* siRNA. A white dotted line in each field was added to visualize the representative outline of the cell. Red arrows indicate cells positive for SA-β-GAL staining. Scale bar, 100 μm. (**D**) Summary data of the percentage of SA-β-GAL-positive cells. N=14 from two independent experiments in each treatment. ***P<0.001 by unpaired Student’s t-test. (**E**) Summary data of cell surface areas. N=8 from two independent experiments. ***P<0.001 by unpaired Student’s t-test. (**F**) Levels of *Cdkn2a* (p16 and p19), *Cdkn1a* (p21), *Il6* and *Serpine1* (PAI-1) mRNA in P2 MEFs transfected with control siRNA or siRNA against *Igfbp5*. N=8-9 in each treatment. *P<0.05, **P<0.01, ***P<0.001 by paired Student’s t-test. (**G**) (Left) Cell proliferation in P2 MEFs transfected with control siRNA and siRNA against *Igfbp5* determined by Cell Counting Kit-8. N=4 in each treatment. **P<0.01 by two-way repeated measures ANOVA with a Student-Newman-Keuls test. (Right) Level of *Igfbp5* mRNA 72 h after transfection. **P<0.01 by paired Student’s t-test. Data are represented as mean +/- SEM.