Within breast cancer—spanning ER-positive (e.g., MCF-7, T47D) and triple-negative models (e.g., MDA-MB-231, MDA-MB-468)—coumarin scaffolds have been evaluated as modulators of aromatase/ER and EGFR signaling, VEGFR-2/CCL2-linked angiogenesis, and the PI3K/Akt/mTOR pathway, with reproducible caspase-mediated apoptosis and G1/G2–M arrest; context-dependent autophagy and MDR/P-gp effects are also noted. This evidence concerns the gene ESR1 and breast cancer.