Abstract: Disclosed are methods and eukaryotic host cells for transgene expression. The cells may be treated and/or modified to increase homologous recombination (HR), decrease non homologous end joining (NHEJ) and/or to enhance a HR/NHEJ ratio in said cell. Such cells can be transfected with vectors comprising the transgene, which advantageously integrates into the genome of the cell to form a concatemeric structure which may comprise more than 200 transgene copies. Certain expression enhancing elements such as MARs are advantageously provided to further enhance and/or facilitate transgene expression. Disclosed is also a recombinant eukaryotic host cell, in particular a non-primate host cell, comprising a transgenic sequence encoding a protein and/or a RNA, in particular a primate protein and/or RNA, involved in translocation across the ER membrane and/or secretion across the cytoplasmic membrane.