BRD4 Inhibitor-18 is a potent BRD4 inhibitor (IC50: 110 nM) with a hydrophobic acetylcyclopentane side chain. BRD4 Inhibitor-18 significantly reduces the proliferation of MV-4-11 cells with high BRD4 levels, inhibits G0/G1 cycle activity, and promotes apoptosis. The inhibition of G0/G1 cycle activity and apoptosis were also observed.