Epitope tags provide a method to localize gene products in living cells, identify associated proteins, track the movement of tagged proteins within a cell, and characterize new genes without creating protein-specific antibodies. The pCMV-Tag1 vector contains both the synthetic FLAG epitopell ll and the c-myc epitope from the human c-myc gene. The pCMV-Tag1 vector allows production of fusion proteins in a variety of conformations. The pCMV-Tag2-5 vectors allow either c-myc or FLAG epitope tagging at either the C- or N-terminus. Each pCMV-Tag2-5 vector is supplied with all three reading frames for easy cloning and expression. The small size of the FLAG (eight amino acids long) and c-myc epitope tags (ten amino acids long) decreases the possibility of interference with the tagged protein. The FLAG and c-myc epitopes are recognized by the anti-c-myc and anti-FLAG antibodies, which can be used to characterize the target protein. The pCMV-Tag vectors allow high-level expression of tagged proteins in mammalian cells. Expression is driven by the human cytomegalovirus (CMV) immediate early promoter. The CMV promoter provides constitutive expression of cloned genes in a wide variety of cell lines. The translational start sequence used in the pCMV-Tag1, pCMV-Tag2 and pCMV-Tag3 vectors is a 10-base Kozak consensus sequence of CGG(A/G)CCATGG. pCMV-Tag4 and pCMV-Tag5 vectors do not contain a translation start sequence. Stable clone selection is made possible with G418 by the presence of the neomycin-resistance gene. The pCMV-Tag vectors are only 4.3 kb, allowing easy cloning and vector manipulations. This small size is due to the single antibiotic selection marker used by both prokaryotic and mammalian cells. The neomycin-resistance gene provides stable selection in mammalian cells with G418 driven by the SV40 promoter and kanamycin selection in E. coli cells driven by the bla (?-lactamase) promoter.