4-Hydroxytamoxifen is an active metabolite of tamoxifen and functions as a selective estrogen receptor modulator (SERM). It is widely used in research settings for its potent anti-estrogenic activity, particularly in studies involving estrogen receptor-positive breast cancer cells. Due to its higher affinity for estrogen receptors compared to tamoxifen, it is employed to inhibit estrogen-dependent cell proliferation in vitro and in animal models. It is also utilized in inducible gene expression systems, such as the Cre-ERT2 system, where 4-hydroxytamoxifen acts as a molecular switch to activate Cre recombinase activity in a temporally controlled manner. This application is critical for conditional gene knockout studies in developmental biology and tissue-specific gene function analysis in transgenic models. Additionally, it serves as a reference compound in pharmacokinetic and metabolic studies aimed at understanding tamoxifen's bioactivation and therapeutic efficacy. Its role in modulating estrogen receptor signaling makes it valuable in screening and developing new SERMs and endocrine therapies.