Analysis of c-Fos protein expression by indirect immunofluorescence and flow cytometry in Jurkat (A through F ) and primary PBMC (G through N). c-Fos protein was determined in 24-hour serum–starved cells before (A, D, G, L) and after a 45-minute treatment with 15% FCS (B, H), 5 μg PHA plus 10⁻⁷ mol/L PMA (E, M), Tat plus 15% FCS (C, I), Tat plus 5 μg PHA plus 10⁻⁷ mol/L PMA (F, N). Horizontal axis, relative c-Fos expression detected by fluorescence intensity (logarithmic scale). Vertical axis, relative number of cells. Nonspecific fluorescence (not shown) was monitored by staining cells with normal rabbit serum plus GAR-FITC.