The mammalian ras gene family comprises three genes, H-ras, K-ras and N-ras.  The ras proteins are a family of GTP-binding and hydrolyzing proteins that regulate cell growth and differentiation.  Overproduction of normal ras proteins or mutations that inhibit their GTPase activity can lead to uncontrolled cell division. 
The transforming activity of ras is dependent on localization of the protein to plasma membranes.  This membrane binding occurs via a series of post-translational modifications of the cytosolic Ras proteins.  The first and mandatory step in this sequence of events is the farnesylation of these proteins.  The reaction is catalyzed by the enzyme farnesyl protein transferase (FPT), and farnesyl pyrophosphate (FPP) serves as the farnesyl group donor in this reaction.  The ras C-terminus contains a sequence motif termed a "Cys-Aaa.sub.1 -Aaa.sub.2 -Xaa" box (CAAX box), wherein Cys is cysteine, Aaa is an aliphatic amino acid, and Xaa is a serine or methionine.  Farnesylation occurs on the cysteinyl residue of the CAAX box (cys-186), thereby attaching the prenyl group on the protein via a thio-ether linkage.