Breast cancer containing estrogen receptors (ER) are responsive to antiestrogen treatment and have a better prognosis compared with ER-negative tumors. The loss of estrogen receptors appears to be associated with a progression to less-differentiated cells. We transfected the human ER into the ER-negative breast cancer cell line MDA-MB-231 cells. We found that expression of adequate ER is strong associated with the ability of human breast cancer cell growth inhibition and progression. Compared with nontransfected or mock-transfected cells, ER-transfected cells exhibited growth slower, forming smaller colonies in soft agar and growth inhibited by estrogen and tamoxifen. Therefore reactivation or transfection of the estrogen receptor gene can be considered as therapeutic approaches to hormone-independent breast cancer.