DNA to enter. Finally, liposome-mediated transformation can be used. The cell surface and the incoming DNA are both negatively charged, so the DNA is coated with lipids. By shielding the DNA and possibly merging with the membrane lipids, these liposomes can facilitate the entry of DNA. Transformation of bacteria, plant cells and animal cells has important research and commercial functions. Targeted introduction of exogenous DNA is used to identify genes because the introduced DNA can cause a mutation or alter the expression of the targeted gene, providing a unique identifying signal. This technology, known as insertion mutagenesis, often employs retroviruses as the vectors of DNA delivery. Such insertion mutagenesis has been often used to identify many oncogenes in specific locations in tumor cells. Transfection Transfection is the process of introducing exogenous DNA into eukaryotic cells. It is a more specific term for animal cells, as the process of carcinogenesis in these cells is also included in the definition of transformation. Typically, transfection describes the changes in a cell's genome due to the introduction of foreign DNA. There are several ways of conducting artificial transfection. Chemical methods involve using chemicals as carriers to introduce DNA, such as calcium phosphate precipitation, DEAE-dextran complexation and lipid-mediated DNA transfer. Physical methods use techniques such as electroporation, microinjection, and cell squeezing to increase the permeability of the cell membrane for accepting DNA. Viral methods (or transduction) use recombinant, lab manipulated viruses as vectors to alter embryos and sperm cells. Transgenesis The use of exogenous DNA to transform cells has spawned the discipline of transgenesis: the use of recombinant DNA techniques to introduce new characters into organisms, mainly through transgenes. A transgene is an introduced DNA segment that be used to encode a gene in its host animal. Biologists uses transgenesis as a tool to breed genetically modified, or transgenic animals that provide a wide range of uses. These include the study of developmental genetics, disease processes and gene regulation. For example, transgenic farm animals can produce human pharmaceuticals alongside increased milk or meat production. Tissues and organs from transgenic animals can also be used in transfusions and transplants with a lesser chance of immune rejection. Sperm cells Using transgenesis to genetically modify animals has spawned a new division of using exogenous DNA to modify sperm cells. Epididymal sperm cells were shown to react to exogenous nucleic acids, allowing for DNA to reversibly bind to the