It is known that the ER+ human breast cancer cell line, MCF7, consists of highly heterogeneous breast cancer cells having significant genetic and phenotypic variations (e.g., differential tamoxifen and letrozole sensitivities) and MCF7-dervided, tamoxifen-resistant (or hormone-independent) sub-lines are widely used as models to study the induction of hormone therapy resistance in ER+ breast cancer (Planas-Silva et al., 2006; Huber-Keener et al., 2012; Zhou et al., 2012). Here, ESR1 is linked to breast cancer.