Abstract: An expression vector for mammalian cells includes a selection cassette with a nucleotide sequence encoding a glutamine synthetase, operably linked to a PGK promoter and a pA signal. The vector may include the EASE element which is known to promote stable integration of the expression cassettes into the genome. The vector also includes a selection cassette with a nucleotide sequence encoding an enzyme that confers resistance against an antibiotic to a bacterial host as a bacterial selection marker, operably linked to a suitable promoter. The vector further includes an expression cassette for a target polypeptide with an insertion site for a nucleotide sequence encoding the target polypeptide, operably linked to a. CMV promoter and a pA signal. The vector also includes a bacterial origin of replication.