Previously, we explored the use of glyceollin, a naturally occurring anti-estrogen from the soy plant, for the treatment of postmenopausal metastatic breast cancer that has transitioned from ER+ to ER- and demonstrated that glyceollin is capable of inhibiting the proliferation and motility in ER- letrozole-resistant breast cancer cells (LTLT-Ca) [7]. This evidence concerns the gene ESR1 and breast cancer.