First, we identified HLA-restricted peptides within the IDO protein to which spontaneous T-cell reactivity were detected in patients suffering from unrelated tumor types, i.e., melanoma, renal cell carcinoma and breast cancer by flow cytometry using HLA/peptide tetramers as well as in ELISPOT assays after in vitro stimulation but also in direct ex vivo assays. This evidence concerns the gene IDO1 and breast cancer.