ESR1 and breast cancer: Previous work using ectopic overexpression of Gata3 in breast cancer cell lines that are ER and Gata3-negative, such as MDA-MB-231, has demonstrated the ability of ectopic Gata3 expression to suppress tumor and metastasis formation, although the relevance of manipulating Gata3 in an ER-negative, non-luminal cell type is uncertain [17–21, 29].