Eukaryotic cells contain 3 different types of prenyltransferases that attach either a farnesyl group (15 carbons) or a geranylgeranyl group (20 carbons) in thioether linkage to C-terminal cysteine residues in a variety of proteins. These posttranslational modifications provide a mechanism for membrane localization of proteins that lack a transmembrane domain. CAAX farnesyltransferase (FTase) attaches a farnesyl group from farnesyl pyrophosphate to cysteine residues at the fourth position from the C terminus of proteins that end in the CAAX box, where C is cysteine, A is usually but not always an aliphatic amino acid, and X is typically methionine or serine. This enzyme has the ability to farnesylate peptides as short as 4 residues in length that conform to the CAAX consensus sequence. The gene for the beta subunit of CAAX farnesyltransferase (FNTB) has been pinpointed to 14q23-q24 by Southern blot hybridization and PCR analyses of panels of human/Chinese hamster somatic cell hybrid lines and by fluorescence chromosomal in situ hybridization.