Simulations of gene therapy in which two marked cells were included among the 300 transplanted HSCs. For these simulations, the apoptosis of unmarked HSC occurred on average once per 50 weeks. The chance of apoptosis for marked cells was one-half that of unmarked HSC. This manipulation led to persistence of a marked clone in most simulations (eg, panels 1c, 1d, 2b, 2c, and 2d). The percentage of marked progenitor cells increased slowly over time in these cases.