Blocking of NO-induced apoptosis in different neoplastic lymphoid cells by the caspase inhibitor zVAD-fmk. Cells were plated at a density of 3 × 10⁵ cells/mL and incubated with 0.2 mmol/L GTN (APO-S and APO-R) or 0.25 mmol/L GTN (CEM, BJAB, BL60, and K50) and/or with 20 μmol/L zVAD-fmk (zVAD). Percentage of apoptotic cells was measured by the DNA-release assay. Data shown are representative of three individual experiments. SD was less than 10%. (□), Control; (▧), +GTN; (▨), +GTN +zVAD; (▧), +zVAD.