<P> Protein production systems (in lab jargon also referred to as' expression systems') are used in the life sciences, biotechnology, and medicine . Molecular biology research uses numerous proteins and enzymes, many of which are from expression systems; particularly DNA polymerase for PCR, reverse transcriptase for RNA analysis, restriction endonucleases for cloning, and to make proteins that are screened in drug discovery as biological targets or as potential drugs themselves . There are also significant applications for expression systems in industrial fermentation, notably the production of biopharmaceuticals such as human insulin to treat diabetes, and to manufacture enzymes . </P> <P> Commonly used protein production systems include those derived from bacteria, yeast, baculovirus / insect, mammalian cells, and more recently filamentous fungi such as Myceliophthora thermophila . </P> <P> The oldest and most widely used expression systems are cell - based and may be defined as the "combination of an expression vector, its cloned DNA, and the host for the vector that provide a context to allow foreign gene function in a host cell, that is, produce proteins at a high level". Overexpression is an abnormally and excessively high level of gene expression which produces a pronounced gene - related phenotype . </P> <P> There are many ways to introduce foreign DNA to a cell for expression, and many different host cells may be used for expression--each expression system has distinct advantages and liabilities . Expression systems are normally referred to by the host and the DNA source or the delivery mechanism for the genetic material . For example, common hosts are bacteria (such as E. coli, B. subtilis), yeast (such as S. cerevisiae) or eukaryotic cell lines . Common DNA sources and delivery mechanisms are viruses (such as baculovirus, retrovirus, adenovirus), plasmids, artificial chromosomes and bacteriophage (such as lambda). The best expression system depends on the gene involved, for example the Saccharomyces cerevisiae is often preferred for proteins that require significant posttranslational modification . Insect or mammal cell lines are used when human - like splicing of mRNA is required . Nonetheless, bacterial expression has the advantage of easily producing large amounts of protein, which is required for X-ray crystallography or nuclear magnetic resonance experiments for structure determination . </P>

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