Pre-B cells can be retrovirally transduced. (A) Pro-B and pre-B cells were isolated from the bone marrow of wild-type mice, transduced with a GFP– containing retroviral vector, and placed in culture. GFP expression was assessed in cell aliquots harvested at 40, 65, and, in an independent experiment, 120 hours later. Cultures were initiated with 1.0 to 1.5 × 10⁵ pro-B or pre-B cells. (B) Ki-67 labeling of pro-B and pre-B cells. Ki-67 expression was measured in these populations after 18 hours culture in vitro.