Farnesyl Transferase (EC 2.5.1.58) (FTase) catalyzes the transfer of a farnesyl isoprenyl group to a cysteine residue fourth from the C-terminus of certain proteins, such as Ras, Rho, Rab, other Ras-related small GTP-binding proteins, and other targets. The enzyme is a zinc metalloenzyme that requires Mg²⁺ for activity. Based on a literature search, a dansyl peptide of the form Dansyl-KTCXXX was selected for further work. The K_m for our selected Dansyl peptide was determined to be 1.14 µM in the presence of 80 nM enzyme. DMSO tolerance studies indicated that DMSO concentrations >1 v% should be avoided. The IC₅₀ of rat FTase for Lonafarnib was determined to be 25.6 nM under the conditions of the assay (10 µM Dansyl peptide, 20 µM Farnsyl Pyrophosphate, and 80 nM FTase). Lonafarnib IC₅₀ Determination To test the suitability of BioAssay System’s Farnesyl Transferase Assay for screening for inhibitors, we performed an initial experiment with a known Farnesyl Transferase inhibitor, Lonafarnib, at 80 nM rat Farnesyl Transferase. Using 10 µM Dansyl substrate and 20 µM Farnesyl Pyrophosphate, we observed an IC₅₀ of 25.6 nM for Lonafarnib. The percent enzyme activity was plotted versus the concentration of Lonafarnib.