['Growth Regulation by Estrogen in Breast cancer (GREB1) was an estrogen receptor (ER) target gene, and GREB1 expression inversely correlated with HER2 status, possibly as a surrogate marker for ER status and a predictor for tamoxifen resistance in breast cancer patients. In the present study, we examine the function and regulation of GREB1 in breast cancer, with the goal to develop GREB1 as a biomarker in breast cancer with de novo and acquired tamoxifen resistance.', 'We overexpressed GREB1 using adenovirus containing the full length GREB1 cDNA (Ad-GREB1) in breast cancer cell lines. The soft agar assay was used as a measure of anchorage independent growth. The effects of GREB1 on cell proliferation in MCF-7 cells transduced with Ad-GREB1 were also measured by the me olic activity using AlamarBlue assay. We tested whether there was interaction between STAT3 and ER, which could repress GREB1 expression by immunoprecipitation assay. The effects of IL-6/JAK/STAT3 cascade activation on estrogen-induced GREB1 promoter activity were determined by luciferase assay and those on gene expression were measured by real time reverse transcription polymerase chain reaction (qRT-PCR).', 'We found that the ability of breast cancer cells to grow in soft agar is enhanced following GREB1 transfection. In MCF-7 cells transduced with Ad-GREB1 or transfected with siRNA GREB1, the metabolic activity was increased or completely abolished, suggesting that GREB1 may function as a growth promoter in breast cancer. E2 treatment increased GREB1 promoter luciferase activity. IL-6 inhibited E2-induced GREB1 transcription activity and GREB1 mRNA expression. Constitutively expressing active STAT3 construct (STAT3-C) dramatically decreased GREB1 transcription.'] Is gREB1 functions as a growth promoter and modulated by IL6/STAT3 in breast cancer?