Expression of foreign genes in Mammalian cells

Many laboratories are currently interested in cloning cellular genes that code for polypeptides that may have academic or therapeutic use. Once the intact gene is isolated, usually in the form of a cDNA clone, it has to be engineered into an expression cassette that is capable of functioning when introduced into a particular host cell. The choice of using a prokaryotic (e.g., bacterial), lower eukaryotic (e.g., yeast), or higher eukaryotic (e.g., mammalian) expression system is frequently dictated by the nature of the polypeptide to be expressed. It is becoming increasingly evident that for the majority of cloned eukaryotic genes there can be considerable problems in achieving reliable expression of biologically active products in bacterial or yeast systems. Although the product may be expressed in high amounts in these systems, often it is not folded correctly and is therefore biologically inactive or is produced in an insoluble, denatured form within occlusion bodies.