The present investigation was carried out with the main aims of (a) comparing the cytotoxicity of a F. leucopyrus decoction to different three breast cancer cell phenotypes, MCF-7 (ER positive / PR positive, Her2 negative), SKBR-3 (ER negative, PR negative, Her2 positive), and MDA-MB-231 (Triple negative), and a non-cancerous MCF-10A breast cell line, and (b) evaluating whether induction of apoptosis and modulation of HSPs (HSPs 70 and 90) are possible mechanisms by which the decoction mediates its anticancer effects in the above cell types. This evidence concerns the gene ESR1 and breast cancer.