In the near future, the nucleotide sequence of the genomes from many different organisms will be available. The next and more challenging step will be to characterize the biological role of each gene and the way in which the encoded protein functions in the cell. Dual-expression vectors for expression of proteins encoded by these genes in mammalian and bacterial cells can be used for this characterization. Typically, eukaryotic genes are expressed in mammalian cells to characterize biological functions and in bacterial cells to facilitate isolation of the protein. This generally requires the use of more than one vector. In contrast, use of a dual-expression vector eliminates the need to subclone from one vector system to another by combining the essential features of both eukaryotic and prokaryotic vectors in a single vector.