OmicronBJECTIVE: To construct mammalian expression vectors for the N- or C-terminal tagging of proteins with a tandem affinity tag comprised of the biotinylatable Avi tag and of a triple FLAG tag. RESULTS: We constructed and tested by transient transfections mammalian expression vectors for the co-expression from a single plasmid of N- or C-terminally tagged proteins bearing a tandem affinity tag comprised of the biotinylatable Avi tag and of a triple FLAG tag separated by a tobacco etch virus (TEV) protease cleavage site, together with a mammalian codon-optimized BirA biotin ligase fused to green fluorescent protein. We also describe platform vectors for the N- or C-terminal AVI-TEV-FLAG tagging of any complementary DNA of choice. These vectors offer versatility and efficiency in the application of metabolic biotinylation tandem affinity tagging of nuclear proteins in mammalian cells.