A method for screening cells wherein, either: a) the screened cells exhibit high level expression of a target protein, and the method comprises: a. introducing into a plurality of host cells a DNA construct that encodes both a target protein and an inhibitor to a selectable marker that is encoded by a polynucleotide that is present in the host-cell prior to the introduction of the DNA construct into the host-cell, wherein the DNA construct is configured to express both the target protein and the inhibitor inside the host cell; b. screening host cells harbouring the DNA construct for the expression of the selectable marker, and c. isolating cells exhibiting reduced expression of the selectable marker, or b) the screened cells are transgene expressing cells which are selected by the method, and the method comprises: a. transfecting cells expressing a fluorescent protein with a vector carrying at least one transgene and an interfering RNA that inhibits the expression of the fluorescent protein; b. measuring fluorescence intensity in the transfected cells; and c. isolating cells having a fluorescence intensity that is lower than the fluorescent intensity of untransfected cells. a) the screened cells exhibit high level expression of a target protein, and the method comprises: a. introducing into a plurality of host cells a DNA construct that encodes both a target protein and an inhibitor to a selectable marker that is encoded by a polynucleotide that is present in the host-cell prior to the introduction of the DNA construct into the host-cell, wherein the DNA construct is configured to express both the target protein and the inhibitor inside the host cell; b. screening host cells harbouring the DNA construct for the expression of the selectable marker, and c. isolating cells exhibiting reduced expression of the selectable marker, or a. introducing into a plurality of host cells a DNA construct that encodes both a target protein and an inhibitor to a selectable marker that is encoded by a polynucleotide that is present in the host-cell prior to the introduction of the DNA construct into the host-cell, wherein the DNA construct is configured to express both the target protein and the inhibitor inside the host cell; b. screening host cells harbouring the DNA construct for the expression of the selectable marker, and c. isolating cells exhibiting reduced expression of the selectable marker, or b) the screened cells are transgene expressing cells which are selected by the method, and the method comprises: a. transfecting cells expressing a fluorescent protein with a vector carrying at least one transgene and an interfering RNA that inhibits the expression of the fluorescent protein; b. measuring fluorescence intensity in the transfected cells; and c. isolating cells having a fluorescence intensity that is lower than the fluorescent intensity of untransfected cells. a. transfecting cells expressing a fluorescent protein with a vector carrying at least one transgene and an interfering RNA that inhibits the expression of the fluorescent protein; b. measuring fluorescence intensity in the transfected cells; and c. isolating cells having a fluorescence intensity that is lower than the fluorescent intensity of untransfected cells.