In vitro differentiation of human NK cells derived from FACS sorter-purified progenitor cells of NOD/SCID mouse BM. (A) Hematopoietic progenitor cells and NK precursor cells were identified in the BM of untreated NOD/SCID mice by staining with CD34/CD38 (top panel) and CD34/CD7 (bottom panel). Cells falling into the gates R1 (CD34⁺CD38^-: 1.5% of total BM cells), R2 (CD34⁺CD38⁺: 12%), R3 (CD34⁺CD7⁺: 0.5%), and R4 (CD34^-CD7⁺: 1.5%) were sorted and cultured for 14 days in NK differentiation medium (see “Materials and methods”). (B) The development of CD56⁺ NK cells in vitro was followed by FACS analysis at the indicated time points.