Fetal BM PreProB-progenitors and ProB-progenitors are committed to the B-lymphoid lineage. (A) Cell frequency at day 14 and day 21 after coculturing flow-sorted fetal BM HSPCs on murine MS-5 stromal cells with cytokines (FLT3L, SCF, IL-7, and IL-2). Data shown as percentage of human CD45⁺ cells (mean - SEM, n = 3-5). (B) Frequency of T cells (CD4⁺ and/or CD8⁺) produced by day 7 and day 14 after coculturing flow-sorted fetal BM HSPCs on OP9-DL1 (GFP⁺) stroma with cytokines (FLT3L and IL-7). Data shown as percentage of GFP^– human cells (mean + SEM, n = 4). (C) B-cell differentiation assay of flow-sorted ELPs cocultured on MS-5 stromal cells with cytokines (FLT3L, SCF, IL-7, and IL-2) showing output on days 3, 7, and 14 (results expressed as mean − SEM, n = 2-4). (D) Myeloid clonogenic assays showing day 14 colony readout from flow-sorted fetal BM HSPCs in methylcellulose colony assays (data shown as mean + SEM, n = 4). (E) Engraftment of fetal BM Lin2^–CD34⁺CD19^–CD10^– progenitors, PreProB-progenitors, and ProB-progenitors in the BM of NSG mice 2 to 3 weeks after transplantation (n = 4 for each progenitor population; total n = 12). Data shown as a percentage of human CD45 (hCD45) cells of all CD45⁺ cells in mouse BM (mean ± SEM). (F) Identity of hCD45⁺ cells engrafted in mouse BM 2 to 3 weeks posttransplant. The plot in the center shows the breakdown of all human CD45⁺ cells, and the plots on either side of the central plot detail the progenitors identified in hCD45⁺CD34⁺ cells.