Downmodulation of CD4 and MHC Class I surface expression on Sup-T1 cells expressing nef after retroviral gene transfer. Bivariate dot plots of flow cytometric measurement of Nef⁻ transduced cells (top) and Nef⁺transduced cells (bottom). CD4-PE and MHC Class I-PE expression versus EGFP expression of Sup-T1 cells 3 days after transduction. Quadrants were set to include 99% of stained nontransduced cells in upper left quadrant. Figures indicate percentage of cells present in corresponding quadrants.