GGTI-286 hydrochloride, a potent GGTase I inhibitor, is 25-fold more potent ( IC 50 =2 μM) than the corresponding methyl ester of FTI-276. GGTI-286 hydrochloride selectively inhibits geranylgeranylation of Rap1A over farnesylation of H-Ras in NIH3T3 cells (IC 50 s =2 and >30 μM, respectively). GGTI-286 hydrochloride also potently inhibits oncogenic K-Ras4B stimulation with an IC 50 of 1 μM.