ESR1 and breast cancer: Both endoxifen and 4-OH-Tam exhibit 30- to 100-fold higher anti-estrogenic potency than NDM-Tam or tamoxifen itself, with respect to their affinity for ER and suppression of estrogen-dependent breast cancer MCF7 cells proliferation, and are considered as the active tamoxifen metabolites responsible for the overall therapeutic drug activity [5–7].