Endoxifen (CAS: 26566-96-7; Molecular Formula: C₂₅H₂₇NO₂; MW: 373.49) is a potent selective estrogen receptor modulator (SERM) and the primary active metabolite of tamoxifen. As a specialty chemical, it is widely used in pharmaceutical research and development for the treatment and prevention of hormone receptor-positive breast cancer. It functions by effectively blocking estrogen receptors in breast tissue, thereby inhibiting the growth of estrogen-dependent tumors. Endoxifen is particularly valuable in personalized medicine for patients who poorly metabolize tamoxifen or do not respond well to it. Ongoing research explores its potential in reducing cancer recurrence risk post-treatment and in other hormone-related conditions. This high-purity reagent (typically ≥98%) is essential for in vitro and in vivo studies on estrogen signaling and oncology.