Plasmid vectors for the expression of coding regions of eukaryotic genes in bacterial, insect and mammalian hosts are in common usage; such expression vectors are frequently used to encode hybrid fusion proteins consisting of a eukaryotic target protein and a specialized region designed to aid in the purification and visualization of the target protein. For example, the pCDM8 expression vector and derivatives thereof encode fusions between the target protein and an 11 amino acid peptide derived from the influenza protein hemagglutinin (HA). The HA epitope tag is useful in Western blotting and immunohistochemical localization of expressed fusion proteins when examined with antibodies raised specifically against the HA-epitope tag.