Breast cancer containing estrogen receptors （ER ） are responslve 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 clifferentiated cells. We transfected the human ER into the ER-negative breast cancer cell line MDA MD 231 cells. We found that expresslon of adequate ER is strong associated with the ability of human breast cancer cell growth inhibition and progression. Compared with nontransfected or mock-trans fected cells

ER-transfected cells exhibited growth slower

forming smaller colonles 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.