Abstract. To establish a model of endocrine resistant breast cancer that is associated with loss of estrogen receptor (ER), MCF7 cells were transfected with several plasmid constructs intended to produce intracellular double stranded hairpin RNA to be processed into siRNA directed against different regions of the ER· mRNA. Stably transformed cells were propagated in long-term culture. One of these lines, designated pII, was selected for further analysis. pII cells exhibited reduced levels of ER· mRNA and protein as well as several estrogen-regulated genes assessed by real-time PCR and were unresponsive to addition of estradiol and tamoxifen.

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