In the early days of biopharmaceutical production, mammalian cells were not commonly used as substrates in the manufacture of bio­technology-derived medicinal products as there were initial safety concerns. For example, transformed mammalian cells (generated via hybridoma technology) have unlimited capacity for continuous population doubling. They are immortal cells and are potentially oncogenic. Hence, there were fears of possible contamination of the eventual biological medicinal product with residual (oncogenic) DNA from the mammalian host cell or substrate. This fear held up the use of mammalian cells as a substrate for many years. Fears were allayed by a 1986 WHO Study Report (and other subsequent scien­tific reports) which concluded that there was no reason to exclude continuous mammalian cell lines for biological production, if the purification process can reduce residual DNA to 10 nanograms per dose or lower. Today, mammalian cells, in particular Chinese ham­ster ovary cells, are the substrates of choice for the manufacture of many therapeutic monoclonal antibodies and biotechnology-derived medicinal products. They produce high yields and possess the ability to perform post-translational modifications, including the produc­tion of therapeutic proteins which are glycosylated. There is also a successful approval history from medicines regulatory authorities involving therapeutic monoclonal antibodies and recombinant pro­teins produced using Chinese hamster ovary cells as the substrate.