dilution) or anti-actin mouse antibody (Sigma, 1:5000 dilution). Anti-rabbit or anti-mouse IgG horseradish peroxidase–linked antibody at 1:3500 dilutions (GE Healthcare) was used as secondary antibodies. Antibodies were detected with chemilumi- nescence reaction using the enhanced chemiluminescence kit (Amersham Biosciences) and visualization with exposure to film. Flow cytometric analysis P1A transgenic mice were sacrificed when moribund. Thymus and spleen tissue were homogenized and passed through a cell strainer (BD Biosciences) to generate a single-cell suspension. Cells were stained in phosphate-buffered saline plus 2% FBS for 20 minutes on ice with the following monoclonal antibodies at a 1:200 dilution, CD4, CD8, B220, CD11c, Gr1, and CD11b (BD Biosciences). Cells were subsequently analyzed by BD LSR II Flow Cytometer. Bromodeoxyuridine (BrdU) incorporation assay Mice were injected intraperitoneally with BrdU (100 mg/kg, Sigma-Aldrich). Then, mice were given BrdU water (1 mg/ml) for 24 hours before sacrifice. BrdU staining kit (BD Biosciences) was used according to the manufacturer’s instructions. Each group had three mice from two independent experiments. References 1. Boon T, van der Bruggen P (1996) Human tumor antigens recognized by T lymphocytes. J Exp Med 183: 725–729. 2. Van den Eynde B, Lethe B, Van Pel A, De Plaen E, Boon T (1991) The gene coding for a major tumor rejection antigen of tumor P815 is identical to the normal gene of syngeneic DBA/2 mice. J Exp Med 173: 1373–1384. 3. Uyttenhove C, Maryanski J, Boon T (1983) Escape of mouse mastocytoma P815 after nearly complete rejection is due to antigen-loss variants rather than immunosuppression. J Exp Med 157: 1040–1052. 4. Ramarathinam L, Sarma S, Maric M, Zhao M, Yang G, et al. (1995) Multiple lineages of tumors express a common tumor antigen, P1A, but they are not cross-protected. J Immunol 155: 5323–5329. 5. 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