SB-743921 (SB-921) is an inhibitor of Kinesin Spindle Protein (KSP), a mitotic protein essential for cell cycle control and motility. It functions by selectively targeting rapidly dividing cells and disrupting spindle formation during cell division, leading to G2/M phase arrest and ultimately causing cell death. It also inhibits proliferation and induces apoptosis in diffuse large B-cell lymphoma (DLBCL) cell lines.