**Figure 3.** G1-S dynamics and model fits in MRC5 cells after IR. (**A**) Relative abundance of EdU positive cells and simulated active Cyclin E/Cdk2 complex (*CycECdk2-a* in panel F). (**B**) Measured and simulated relative total p21 abundance (*p21* in F).(**C**) Measured and simulated relative total Cyclin E1 abundance (*CycE*+*CycE/Cdk2*+*CycE/Cdk2-a* in panel F). (**D**) Measured and simulated relative total Cdk2 abundance (*Cdk2*+*CycE/Cdk2*+*CycE/Cdk2-a* in panel F). (**E**) Measured and simulated relative phosphorylated (Thr160) Cdk2 abundance (*CycECdk2*+*CycECdk2-a* in panel F). (**F**) Wiring scheme of the best approximating p21-dependent G1-S transition model. (**G**) Steady state analysis of active Cdk2 (*CycE/Cdk2-a* in F of the parameterized combined DNA damage-G1-S arrest model (Figure S4) as a function of DNA damage response (DDR), i.e. γH2AX foci, including free parameter perturbations by sampling 50 times from a uniform distribution within an interval of plus/minus 20% around the original parameter value. Solid line: Stable steady state of *CycE/Cdk2-a* of the parameterized model as a function of DNA damage (DDR). Light gray region: 5-95% of stable steady states of *CycE/Cdk2-a* of the parameterized model with perturbed free parameters. Dark gray region: First to third quartile of steady states of *CycE/Cdk2-a* of the parameterized model with perturbed free parameters. **Inset**: Steady state γH2AX foci, i.e. BASE+TAF from Figure S4, as a function of IR [Gy]. **A**-**D**: Lines indicate simulations of the fitted model. Symbols indicate mean measured values ± SEM (n≥3) scaled to day 0. Representative Western Blots are shown in Figure S6, Supplemental Figures. The corresponding data are provided in Supplemental Data Sets 1-13.