Several cell lines have been characterized which, being derived from the different breast cancer subtype specimens, enable modeling of either subtype in in vitro studies and as the tumor xenografts growing in nude mice: MCF-7 and T47D cells corresponding to luminal A (ER and PR positive, HER2 negative), BT474 cells corresponding to luminal B (ER positive, HER2 positive), MDA-MB-231, MDA-MB-468, and BT549 cells corresponding to triple negative, etc. [5]. The gene discussed is ESR1; the disease is breast cancer.