Simulation of expanded cord blood cell transplantation in humans. The average numbers (Avg Pop) of short-term repopulating cells (STRCs) remaining after 1 year are shown. Also shown is the dependency of this number on the length of time an STRC-derived clone contributes to blood cell production and the initial number of STRCs transplanted. Results derive from 500 independent simulations for each set of parameters. When the mean length of time a STRC-derived clone contributes to blood cell production is set at 12 weeks, rare clones often persist for 1 year or longer because of the stochasticity in expansion and differentiation decisions of progeny cells. Indeed, the plot indicates persistence whenever the mean STRC-derived clone lifespan is over 10 weeks and over 200 to 500 STRCs are transplanted. Thus, the argument that observing a cell at 1 year implies that it derived from transplanted HSCs is not secure.