Post-translational protein prenylation is a 3-step process that occurs at the C-terminus of a number of proteins involved in cell growth control and oncogenesis. Isoprenylcysteine carboxyl methyltransferase (Icmt) catalyzes the methylation of C-terminal prenylcysteine residues, the last step in this process. Cysmethynil is an indole-based, time-dependent inhibitor of Icmt (IC50 = adenosylmethionine (Ki = 0.14 µM for the final complex). It does not inhibit other enzymes in the prenylation pathway (farensyltransferase, geranylgeranyltransferase type I, and Rce1) at concentrations up to 50 µM, or related methyltransferases. Treatment of cancer cells results in a dose-dependent decrease in Ras carboxylmethylation, mislocalization of Ras, and impaired signaling through Ras pathways. Treatment of PC3 prostate cancer cells with 25 µM cysmethynil resulted in decreased mTOR signaling, accumulation of cells in the G1 phase, and autophagy-mediated cell death.