To evaluate the cytotoxic specificity of LT-IIc towards various types of human breast cancer cells, we compared its effects on TNBC human breast cancer cell lines MDA-MB-231 and BT549, the ER+ breast cancer cell lines MCF-7 and T47D, the HER2+ breast cancer cell SKBR3, and MCF10A, an immortalized, but not transformed, breast epithelial cell line. This evidence concerns the gene ESR1 and breast cancer.