SB743921 HCl is primarily used in research settings as a potent and selective inhibitor of the kinesin spindle protein (KSP), also known as Eg5. This protein plays a critical role in cell division, specifically during the formation of the mitotic spindle. By inhibiting KSP, SB743921 HCl disrupts the mitotic process, leading to cell cycle arrest and apoptosis in rapidly dividing cells. This mechanism makes it a valuable tool in studying cancer biology, as it helps researchers understand the role of KSP in tumor cell proliferation and explore its potential as a therapeutic target. Additionally, SB743921 HCl is utilized in preclinical studies to evaluate its efficacy in treating various cancers, particularly those resistant to conventional therapies. Its ability to induce cell death in cancer cells without affecting non-dividing cells highlights its potential for developing targeted anticancer treatments.