Acquisition gate used to define CD34⁺CD38⁻ cell population. Region R1 was defined as the lymphoid gate containing small, agranular cells, as previously published.7 Quadrants are defined by fluorescein isothiocyanate (FITC) and PE-labeled isotype controls. Region R2 was used to define CD34⁺CD38⁻ cells for FACS acquisition. This region contains CD34⁺ cells with PE-CD38 fluorescence less than half of the isotype control.