S-trityl-L-Cysteine is a non-natural amino acid and an inhibitor of Eg5, also known as KSP and Kif11, a mitotic kinesin necessary for mitotic spindle formation. S-trityl-L-Cysteine inhibits the ATPase activity of Eg5 in basal and microtubule-stimulated states (IC50s = 1,000 and 140 nM, respectively). It is selective for Eg5 over nine other human kinesins in an enzyme-coupled assay. It reversibly inhibits Eg5-driven microtubule sliding velocity with an IC50 value of 500 nM using X. laevis recombinant Eg5. It induces cell cycle arrest in HeLa cells (IC50 = 700 nM), reversibly halting the cell cycle in the mitotic phase by inhibiting the separation of duplicated chromosomes and preventing bipolar spindle formation. S-trityl-L-Cysteine inhibits the growth of cancer cells in vitro when tested against the National Cancer Institute (NCI) 60 human cancer cell line panel (average GI50 = 1.3 μM) and in mouse xenograft models.