Tolerance to tumor-nonmutated self proteins represents a major obstacle for successful cancer immunotherapy. Since this tolerance primarily concerns dominant epitopes, we hypothesized that targeting cryptic epitopes that have a low affinity for HLA could be an efficient strategy to breach the tolerance to tumor Ags. Using the P1Y heteroclitic peptide approach, we identified low affinity cryptic HLA-A* 0201-restricted epitopes derived from two widely expressed tumor Ags, HER-2/neu and hTERT. The P1Y variants of four HER-2/neu (neu 391, neu 402, neu 466, neu 650)-and two hTERT (hTERT 572 and hTERT 988)-derived low affinity peptides exhibited strong affinity for HLA-A* 0201 and stimulated specific CTL from healthy donor PBMCs. These CTL specifically recognized HER-2/neu-and hTERT-expressing tumor cells of various histological origins. In vivo studies showed that HLA-A* 0201 transgenic HHD mice vaccinated with the P1Y variant peptides generated CTL that specifically lysed Ag-expressing tumor cells, thus recognizing the cognate endogenous Ags. These results suggest that heteroclitic variants of low affinity, cryptic epitopes of widely expressed tumor Ags may serve as valid tools for tumor immunotherapy.