(E/Z)-Endoxifen is an active metabolite of tamoxifen produced by the sequential action of cytochrome P450 (CYP) isoforms, including CYP2D6. It is a strong anti-estrogen, as it has an approximately 100-fold greater affinity for estrogen receptors than tamoxifen. As the efficient conversion of tamoxifen to endoxifen depends on CYP2D6, polymorphisms in this CYP isoform can impact the effectiveness of TMX treatment.