The primary objective of this article was to investigate the effect of GR on the telomerase activity and on its inhibition by BIBR1532 in two breast cancer cell lines, the highly invasive and metastatic triple-negative (ER-, PR-, Her2-) MDA-MB 231 and the less invasive Estrogen-dependent (ER+) MCF-7 cells. This evidence concerns the gene ESR1 and breast carcinoma.