Although other cells lines, such as African green monkey kidney (COS) and baby hamster kidney (BHK), can be used for recombinant protein production, the most commonly employed host system in transient expression of mammalian cells involves derivatives of the HEK293 cell line, which is based on the human embryonic kidney cell line established in 1977 by Graham et al. The HEK293 cell line was created via transformation with sheared Adenovirus 5 DNA. Advantages of using this cell line include their high rates of transfection and ability to grow in a serum-free medium, which results in reduced cost and lowered risk of contamination with animal-derived material typically found in serum.
Several engineered sublines were later developed by incorporating viral elements derived from mammalian viruses, such as SV40 virus or Epstein–Barr virus (EBV), which are notable for their high retention of plasmid DNA in an episomal state and their capacity to increase transcription and translation via specific viral properties. These later sublines were consequently identified to have two interacting components: the SV40 large T-antigen binding to the SV20 origin of replication (SV40ori) and the EBV-derived nuclear antigen-1 (EBNA-1) protein to its associated origin of replication (oriP).
Typical historical yields of transient expression in HEK293 cells transfected using PEI-25kDa  was 20-40 mg/L of recombinant antibody protein. In 2008, Backliwal et. al reported for the first time yields crossing 1 g/L of recombinant antibody protein.