Donor-type precursor levels in the progeny of different types of HSCs. Shown are the mean levels of donor-type precursors (± SE). The number of experiments (n), each using an individual animal repopulated by an independent balanced, My-bi, or Ly-bi HSC, is indicated below the x-axis. Data from unmanipulated animals (control) are shown for comparison. (A) Lymphoid and myeloid stroma. (B) CFUc. Note that panels A and B use different scales for the y-axis. Lymphoid stroma indicates the frequency of precursors that respond to stroma in Whitlock-Witte conditions to generate colonies of pre-B cells. The cell that repopulates the stroma to differentiate into pre-B cells and B cells is arguably the earliest B-lineage precursor.²⁶  Myeloid stroma indicates the frequency of cells that respond to stroma in Dexter conditions to generate colonies of myeloid cells. This assay is identical to the long-term culture–initiating cell (LTC-IC) assay.^9,40  However, the relationship between LTC-ICs and repopulating HSCs in previously transplanted BM has not been defined. Therefore, we chose to use a conservative interpretation of the data to indicate myeloid differentiation potential. CFUc indicates colony-forming units in response to IL-3. Individual colonies from all experiments were harvested and genotyped to determine the fraction of colonies derived from the transplanted HSC clone. The level of significance compared with precursors from balanced HSCs is indicated.