HER-2/neu (neu-N) transgenic mice were purchased from Jackson Laboratory (FVB-Tg(MMTV-Erbb2)NK1Mul/J), and bred and housed at the Johns Hopkins animal facility. Experiments were performed using female mice between 6 and 12 weeks old and protocols approved by the Animal Care and Use Committee of the Johns Hopkins University School of Medicine. High and low avidity T cell receptor (TCR) transgenic mice were generated from RNEU420-429 specific T cell clones as described (22 (link)). Dilutional tetramer staining was used to verify the avidity of each mouse before use.