A cell line screening method comprising, (a) providing a plurality of host cells transfected with a plurality of DNA constructs, wherein each host cell integrates the DNA construct to make a library of constructs in the plurality of host cells, and wherein the DNA constructs comprise a nucleic acid encoding a protein of interest, a nucleic acid encoding a reporter, a first DNA recombinase recognition sequence and a second DNA recombinase recognition sequence, wherein the first DNA recombinase recognition sequence is located upstream of the nucleic acid encoding the reporter and the second DNA recombinase recognition sequence is located downstream of the nucleic acid encoding the reporter whereby a DNA recombinase is capable of removing the nucleic acid encoding the reporter leaving the gene of interest intact, wherein the nucleic acid encoding the protein of interest is operably linked to a promoter, and wherein the nucleic acid encoding the reporter is also operably linked to the promoter; (b) culturing the transfected cells in a cell culture media wherein the reporter is expressed; (c) screening the transfected host cells for the reporter, and selecting the host cells expressing a high level of the reporter whereby the host cells expressing a high level of the protein of interest are also selected; (d) exposing the selected host cells to the DNA recombinase wherein the DNA recombinase acts at the DNA recombinase recognition sequences whereby the cells no longer express the reporter and the cells continue to express the protein of interest; and (e) confirming the cells express a desired amount of the protein of interest. (a) providing a plurality of host cells transfected with a plurality of DNA constructs, wherein each host cell integrates the DNA construct to make a library of constructs in the plurality of host cells, and wherein the DNA constructs comprise a nucleic acid encoding a protein of interest, a nucleic acid encoding a reporter, a first DNA recombinase recognition sequence and a second DNA recombinase recognition sequence, wherein the first DNA recombinase recognition sequence is located upstream of the nucleic acid encoding the reporter and the second DNA recombinase recognition sequence is located downstream of the nucleic acid encoding the reporter whereby a DNA recombinase is capable of removing the nucleic acid encoding the reporter leaving the gene of interest intact, wherein the nucleic acid encoding the protein of interest is operably linked to a promoter, and wherein the nucleic acid encoding the reporter is also operably linked to the promoter; (b) culturing the transfected cells in a cell culture media wherein the reporter is expressed; (c) screening the transfected host cells for the reporter, and selecting the host cells expressing a high level of the reporter whereby the host cells expressing a high level of the protein of interest are also selected; (d) exposing the selected host cells to the DNA recombinase wherein the DNA recombinase acts at the DNA recombinase recognition sequences whereby the cells no longer express the reporter and the cells continue to express the protein of interest; and (e) confirming the cells express a desired amount of the protein of interest. <EOS>