The identification of increased endoxifen effectiveness as an anti-estrogenic agent in the setting of ERβ is of significant clinical importance due to the fact that ERβ expression is reported to exist in approximately 75% of invasive breast cancers [33,36,37,42,62-64] and in a subset of tumors which are ERα negative [41,45,65]. This evidence concerns the gene ESR1 and invasive breast carcinoma.