FLAG expression was detected in mouse BM lineage-negative (Lin^–) cells transduced with SV(Nef-FLAG) in vitro. Mouse Lin^– BM cells were prepared and transduced in the absence of cytokines, with SV(Nef-FLAG) (A-B) or SV(BUGT) (negative control; C-D) as described in “Materials and methods.” They were cultured for 7 or 42 days, then subjected to intracellular immunostaining to detect FLAG expression. Results are shown here for the 7-day time point. Immunofluorescence photomicrographs (original magnification, × 200) are shown next to phase contrast photomicrographs of the same fields. Sporadic background autofluorescence is seen in the control cultures (D). Results for the 6-week assay point were comparable (data not shown).