The goal was achieved by the development of the CEMAX system that allows site-specific integration of expression cassettes at highly active sites of transcription. Producer cells were generated using a DNA double-strand break induced homologous recombination mechanism based on recombinant CEMAX host cells. This enabled the rapid and reproducible establishment of stable producer cell lines with known characteristics. Productivities of up to 10 pg per cell and day could be achieved even for the production of highly glycosylated proteins. In a separate aspect of this work CHO-K1 host cells growing in suspension in serum-free medium were improved to reach viable cell densities of 107 cells per ml an allow cloning efficiencies up to 71 % in serum-free medium.
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