Effects of PS-341 on apoptosis in transplanted Tax tumors. (A) Control animals or animals treated with 0.1 mg/kg PS-341 (subcutaneously) or drug-treated mice with no tumor transplants were killed at 8 weeks of treatment. Apoptosis was measured on paraffin-embedded sections by TUNEL staining as described in “Materials and methods.” Levels of apoptosis were significantly higher in PS-341–treated tumors compared with controls. Analysis of DNA fragmentation is shown for 3 representative fields obtained from PS-341–treated tumors (i-iii) or vehicle-treated tumors (iv-vi). (B) The apoptotic index was calculated by dividing the number of TUNEL⁺ cells by the total number of cells in 6 random fields in 3 tumors. The statistical significance of apoptosis in response to PS-341 treatment was evaluated by a paired t test: P ≤ .002 for untreated compared to PS-341–treated tumors. Error bars correspond to SEM.