Fold increase in tumor volume of PS-341–treated and untreated tumors in Tax transgenic animals. Tax-positive and Tax-negative mice were injected with drug or vehicle thrice weekly via the subcutaneous route as described in “Materials and methods.” Tumor volumes were measured on Tax tumor–positive mice every other week for 8 weeks during the treatment using vernier calipers. The fold increase in tumor volume at 0 week at the start of treatment compared to that at different time intervals is depicted here for 0, 2, 4, 6, and 8 weeks of treatment. Tax-negative mice received drug as a control for toxicity. The numbers correspond to different mice with tumors treated with either PS-341 or vehicle. Mice exhibiting tumor volume increases of 25% or more of initial volume were considered to be tumor progressors, those with 50% or more overall decrease were considered to be partial remissions, and all forms of tumors in between indicate a stable disease. The statistical significance of the response to PS-341 treatment on tumor growth was evaluated by a paired t test: P ≤ .9 for vehicle treated compared to PS-341 treated groups.