DNA transfection is the process by which DNA is taken up by eukaryotic cells. While the application of bacterial transformation is typically for overexpression of genes in an effort to produce large quantities of proteins that can be purified and studied, DNA transfection is typically used for the studying the effect of gene expression in a cellular context. In DNA transfection, DNA introduced into the cell can either be expressed on an extrachromosomal plasmid (called transient transfection), or integrated in to the cellular genome (called stable transfection). Transient transfection is beneficial for high level of expression, due to the presence of multiple copies of the gene cassette, and quick analysis (within 24 to 96 hours). Stable transfection is advantageous for long term studies of gene expression in the cellular context. Transfection methods are designed to overcome repulsive forces that hinder delivery of DNA or RNA molecules (in which the backbone is negatively charged) across the cell membrane (which also contains negative charges). Transfection methods typically include the mixing of nucleic acids with positively charged reagents that neutralize the negative charges of the DNA or RNA backbone. The mixture is composed on complexes that more readily cross the cell membrane. Physical methods such as electroporation and particle bombardment can also be used for transfection.