Therefore, EGCG can be considered a phytoestrogen due to its structural similarity to E2, a characteristic that allows it to generate estrogenic or anti-estrogenic effects after binding to Erα receptors [18], inhibiting the growth of several ER-negative breast cancer stem cells, such as SUM-149, SUM-190, and MDA-MB-231, in which ER-α receptor variants are expressed such as ER-α36, causing a reduction in the expression of such receptors. The gene discussed is ESR1; the disease is breast cancer.