EPZ015666 is a potent and selective inhibitor of the protein arginine methyltransferase 5 (PRMT5), an enzyme involved in epigenetic regulation through methylation of arginine residues on histone and non-histone proteins. Its primary application lies in cancer research, where it is used to study the role of PRMT5 in tumor growth and progression. Elevated PRMT5 activity has been linked to several cancers, including lymphoma, leukemia, and certain solid tumors, making EPZ015666 a valuable tool for probing therapeutic strategies that target epigenetic pathways. In preclinical studies, EPZ015666 has demonstrated antiproliferative effects in cancer cell lines by inducing cell cycle arrest and apoptosis, particularly in cells dependent on PRMT5 function. It is widely used in vitro and in vivo to validate PRMT5 as a drug target and to understand the downstream effects of its inhibition on gene expression and cellular signaling. Due to its selectivity and pharmacological properties, it also serves as a lead compound for the development of clinical-grade PRMT5 inhibitors. Researchers use EPZ015666 to explore combination therapies with other anticancer agents, aiming to enhance treatment efficacy and overcome drug resistance.