ESR1 and breast cancer: To assess the relevance of DLL1 in BC, we first examined the expression levels of DLL1 in three different tumorigenic BC cell lines: poorly aggressive luminal A MCF-7 and luminal B BT474 cells, that express the estrogen receptor and retain an epithelial morphology, and the highly aggressive, invasive and poorly differentiated TNBC MDA-MB-231 cells [19].