Ritonavir decreases apoptosis in normal CD4⁺ T cells deprived of IL-2 using the TdT assay. PBMC from normal donors prepared and cultured as described in Fig 3 demonstrate decreased apoptosis as measured by the TdT assay after exposure to ritonavir. The number of apoptotic cells was determined using 2-color flow cytometry. Cells were gated to include only CD4⁺ cells (stained with FITC-labeled CD4-PE MoAb) are seen in the histogram above.