Direct microscopic visualization of genetically modified PB cell progeny using the GFP marker. Photomicrographs (original magnification × 2,000) of the identical field from a PB film obtained from an animal transplanted with GFP-transduced marrow both before (B), and after, staining with Wright-Giemsa (A) are shown. The arrowhead denotes a granulocyte exhibiting bright fluorescence while the arrows mark a very weakly fluorescent granulocyte (top arrow) and a mononuclear cell lacking fluorescence (bottom arrow).