FLAG expression is detected for more than 13 months in the peripheral blood (PB) of SV(Nef-FLAG) recipients. Rats were injected intrafemorally with SV(Nef-FLAG) or SV(BUGT), as a control, at doses of 2 × 10¹¹ IU/rat. PB obtained at 2- to 4-week intervals after injection was treated with NH4Cl to lyse red blood cells, then stained with anti-FLAG antibody. FLAG expression was detected by flow cytometry, and data were analyzed as described in “Materials and methods.” (A) Time course of FLAG expression in PB leukocytes of all rats injected with SV(Nef-FLAG). Background positivity (ie, that seen in control rats' blood) was subtracted. (B) FACS histogram analyses for FLAG expression level in PB leukocytes from representative control and test rats 2 weeks after injection: (i), control rat; (ii), SV(Nef-FLAG) recipient rat no. 1.