FLTX1 is a fluorescent Tamoxifen derivative that can specifically label intracellular Tamoxifen-binding sites (estrogen receptors) under permeabilized and non-permeabilized conditions. FLTX1 exhibits the potent antiestrogenic properties of Tamoxifen in breast cancer cells. FLTX1 is devoid of the estrogenic agonistic effect on the uterus.