4-Hydroxytamoxifen is primarily used in research and medical applications due to its role as an active metabolite of tamoxifen. It is widely employed in studies focusing on estrogen receptor modulation, particularly in breast cancer research, where it helps in understanding the mechanisms of estrogen receptor-positive cancers. Additionally, it is utilized in laboratory settings to induce gene expression in genetically modified systems, such as the Cre-loxP system, enabling precise control over gene activation or inactivation in experimental models. Its ability to selectively bind to estrogen receptors makes it a valuable tool in both therapeutic development and molecular biology research.