Abstract: The present invention is directed to a method for inactivation of expression of a gene in a eucaryotic cell comprising (i) transfection of a eucaryotic cell with DNA comprising an expression cassette for expression of a cell surface protein and an expression cassette for expression of a RNAi compound, said compound being capable of inactivating expression of said gene, wherein said expression cassette for expression of a cell surface protein and said expression cassette for expression of a RNAi compound are located on the same vector DNA, and (ii) enrichment and/or selection of cells which express said cell surface protein.