A linearized vector nucleic acid suitable for expressing at least one polypeptide of interest in a mammalian cell, comprising (a) at least one expression cassette suitable for expressing a polypeptide of interest, wherein said expression cassette comprises at least one promoter or promoter/enhancer element; (b) an expression cassette comprising a mammalian selectable marker gene, wherein said mammalian selectable marker gene is an antibiotic resistance gene and wherein said expression cassette comprises at least one promoter or promoter/enhancer element; (c) an expression cassette (MASM) comprising a mammalian amplifiable, selectable marker gene encoding an enzymatically functional dihydrofolate reductase (DHFR), wherein said expression cassette comprises at least one promoter or promoter/enhancer element; and (d) an expression cassette comprising a prokaryotic selectable marker gene, wherein the expression cassette for expressing the polypeptide of interest comprises a stronger promoter and/or enhancer than the expression cassettes for expressing the selectable markers, and wherein the expression cassette is flanked 5' by the expression cassette (MASM), the expression cassette is located 3' from the expression cassette and wherein the expression cassettes (MASM), and are arranged in the same 5' to 3' orientation, and wherein said vector has been linearized via a unique linearization restriction site that is located in the circular form of the vector between the expression cassettes and (MASM) and wherein in said circular form of the vector, the expression cassette is located between the expression cassettes and (MASM), so that in the linearized vector, the expression cassette is flanked 5' by the expression cassette (MASM) and 3' by the expression cassette and the expression cassette is located 3' of the expression cassette, and wherein the linear vector nucleic acid comprises the following genetic elements in the indicated arrangement, wherein the 5' to 3' direction is indicated by the →: I. Promoter of the (MASM) expression cassette (→) II. Gene encoding the mammalian amplifiable selectable marker of the (MASM) expression cassette (→) III. Intron of the (MASM) expression cassette (→) IV. PolyA site of the (MASM) expression cassette (→) V. Promoter of the expression cassette (→) VI. Intron of the expression cassette (→) VII. Polynucleotide encoding a polypeptide of interest, which is inserted in the expression cassette (→) VIII. PolyA site of the expression cassette (→) IX. Promoter of the (POI') expression cassette (→) X. Intron of the (POI') expression cassette (→) XI. Polynucleotide encoding an additional polypeptide of interest, which is inserted in the (POI') expression cassette (→) XII. PolyA site of the (POI') expression cassette (→) XIII. Promoter of the expression cassette (→) XIV. Gene encoding the mammalian selectable marker of the expression cassette (→) XV. PolyA site of the expression cassette (→) XVI. PSM expression cassette (→) or (←), wherein the expression cassette POI' as indicated by elements IX to XII is optional. (a) at least one expression cassette suitable for expressing a polypeptide of interest, wherein said expression cassette comprises at least one promoter or promoter/enhancer element; (b) an expression cassette comprising a mammalian selectable marker gene, wherein said mammalian selectable marker gene is an antibiotic resistance gene and wherein said expression cassette comprises at least one promoter or promoter/enhancer element; (c) an expression cassette (MASM) comprising a mammalian amplifiable, selectable marker gene encoding an enzymatically functional dihydrofolate reductase (DHFR), wherein said expression cassette comprises at least one promoter or promoter/enhancer element; and (d) an expression cassette comprising a prokaryotic selectable marker gene, I. Promoter of the (MASM) expression cassette (→) II. Gene encoding the mammalian amplifiable selectable marker of the (MASM) expression cassette (→) III. Intron of the (MASM) expression cassette (→) IV. PolyA site of the (MASM) expression cassette (→) V. Promoter of the expression cassette (→) VI. Intron of the expression cassette (→) VII. Polynucleotide encoding a polypeptide of interest, which is inserted in the expression cassette (→) VIII. PolyA site of the expression cassette (→) IX. Promoter of the (POI') expression cassette (→) X. Intron of the (POI') expression cassette (→) XI. Polynucleotide encoding an additional polypeptide of interest, which is inserted in the (POI') expression cassette (→) XII. PolyA site of the (POI') expression cassette (→) XIII. Promoter of the expression cassette (→) XIV. Gene encoding the mammalian selectable marker of the expression cassette (→) XV. PolyA site of the expression cassette (→) XVI. PSM expression cassette (→) or (←),