The role of estrogens as promoters of breast carcinogenesis has been well established while their action in tumor invasion appears more complex. Breast cancer cells without estrogen receptor (ER) are generally less differentiated and more aggressive than those containing function ER. Moreover the reexpression of ER by transfection in ER-negative cell lines inhibit their metastatic potential. These results suggest a protective role of ER in the metastatic progression of breast cancers. Studies of the underlying mechanisms of this effect may open new therapeutical strategies.