CD34 cells, but not CD4 T cells, were transduced using the stem cell transduction protocol. The stem cell transduction protocol is described in “Materials and methods.” Cultured CD34 cells were exposed to the YFP transgene-containing oncoretrovirus (A). In parallel, cultured T cells were exposed to the same oncoretrovirus (B). Flow cytometry was performed 24 hours after the last exposure to the oncoretrovirus. The histogram on the right is gated on CD3⁺CD4⁺ lymphocytes. In 4 of 4 experiments, the percentage of YFP⁺ CD4 T cells was not greater than our detection limit of 0.05%, determined using negative controls (not shown).