For each R_0, 20 paths were simulated using the best human parameter estimates: λ = 1 per 40 weeks, ν = 1 per 56.1 weeks, α = 1 per 285.7 weeks, and μ = 1 per 6.7 weeks. Values for R (HSCs, compartment 1 in Figure 1) and C (contributing clones, compartment 2 in Figure 1) were recorded over time. When R₀ = 50, the number of contributing clones (C) increased to more than 10 in 80% of the simulated transplantations but did not do so until weeks 23 to 95. When R₀ = 100 and R₀ = 200, this target is accomplished in 50% and 100% of the simulations, respectively, by week 10. However, the numbers of clones contributing to blood cell production remain less than 25 and 52 at 100 weeks. Throughout this time, contributing clones must undergo significant expansion to generate adequate numbers of mature blood cells. The kinetics help explain why patients are particularly vulnerable to other marrow insults, such as drugs or chemotherapy, long after transplantation and despite normal complete blood count values. Our data also support the argument that the time to marrow recovery reflects the robustness of HSC engraftment.⁴²