High levels of Th2 cell-derived cytokines were detected in tumor sites of patients with breast cancer, with the levels of IL-4 and the amount of tumor-infiltrating CD4+ T cells being positively correlated with tumor progression, as well as with metastasis to sentinel lymph nodes,15,355,356 highlighting the clinical relevance of Th2 cells in the pathogenesis of breast tumors. This evidence concerns the gene IL4 and breast cancer.