NK cell and BL differentiation potential of UCB CD34⁺ HPC populations. Purified UCB CD34⁺ cells were sorted into CD45RA^intCD7^- (R1), CD45RA^hiCD7^- (R2), CD45RA^hiCD7⁺ (R3), CD45RA^intCD7⁺ (R4), and CD45RA^-CD7⁺ (R5) populations and cultured for 21 days with SCF, FL, IL-2, IL-7, and IL-15 (NK condition without MS5). Alternatively, cells were seeded onto MS5 cells with SCF, TPO, and IL-7 and cultured for 14 days (B condition). At the end of the cultures, cells were harvested and labeled with CD56-PE and CD19-FITC mAbs before FACS analysis: histograms and the indicated percentages of specifically labeled cells are based on control mAb labeling. In vitro-generated NK cells were homogeneously CD8⁺CD56⁺, whereas a minority of CD10⁺CD19⁺ BLs coexpressed CD20. At culture initiation none of the populations tested contained CD56⁺ cells; CD34⁺CD19⁺ pre-B cells represented 6% ± 2% (n = 3) of sorted CD45RA^hiCD7^- (R2) cells. Data are from 1 of 2 experiments.