The progression of human breast cancer is often associated with a loss of estrogen dependence for growth, a resistance to estrogen antagonists such as tamoxifen, and the metastatic spread of the disease to secondary sites. Cell lines developed from such advanced breast tumors are often metastatic in athymic mice, show a loss of estrogen receptor mRNA and protein (ER−), and do not respond to 17β-estradiol. However many advanced human breast tumors do express significant amounts of ER transcript, especially when analyzed by more sensitive methods of detection including RT-PCR and Ribonuclease Protection Assay (RPA). No metastatic, ER+breast tumor cell line has previously existed to examine the role of ER in metastatic progression and acquired drug (tamoxifen) resistance. The GI-101A cell line was recently developed from a metastatic breast tumor xenograft and is both tumorogenic and metastatic to the lungs and lymph node when injected into athymic mice, a pattern similar to that seen in patients. While Western blot analysis initially indicated that GI-101A was ER−, analysis of ER mRNA by RT-PCR and RPA have demonstrated the expression of ER (as well as EGF receptor and neu oncogene) transcripts. Functional ER in GI-101A was confirmed by a clear growth response to 17β-estradiol in culture. Optimal 17β-estradiol concentrations were significantly lower for GI101A than for MCF-7 (1nmas opposed to ≥10nm), and GI-101A growth was inhibited at 17β-estradiol concentrations above 10nm. Unlike MCF-7 cells, GI-101A shows constitutive expression of pS2 protein in hormone depleted media with no apparent induction by 17β-estradiol supplimentation, as well as a resistance to the anti-estrogen tamoxifen at concentrations up to 10nm. Finally, ER transcripts which likely represent an alternately spliced ER variant which has previously been shown to encode a constitutively active ER protein have been detected in GI-101A at levels similar to the wild type transcript, and offer a possible mechanism for estrogen independence, tamoxifen resistance, and constitutive pS2 expression.