Flow cytometric analysis of GFP expression in GP + E86 cells transduced with amphotropic MGirL22Y vector particles after 7 days of selection with 100 nmol/L trimetrexate. The broken line profile designates the fluorescence of parental GP + E86 control cells while the solid line profile indicates the fluorescence of the GFP-transduced population. The proportion of cells sorted for high level GFP expression is indicated.