We have noticed in our general testing of the ability of various organotin polyethers to inhibit different cancer cell lines that in certain cases the human breast cancer cell line MDA-MB-231, that is estrogen-independent, estrogen receptor negative, showed better test results than the breast cancer cell line MCF-7, a well-characterized estrogen receptor positive control cell line (cells are positive for cytoplasmic estrogen receptors) and therefore are a useful in vitro model of breast cancer to study the role of estrogen in breast cancer. Here, ESR1 is linked to breast cancer.