The study of gene expression regulation relies on the introduction of foreign DNA into eukaryotic cells. A wide variety of DNA-transfer procedures have been developed that utilize retroviruses (1 ), polycations (2 ), liposomes (3 ), chromosomes (4 ), reconstituted viral envelopes (5 ) and other chemical reagents, such as calcium phosphate (6 ), DEAE-dextran (7 ), and lipopolyamines (8 ). DNA also can be transferred into cells by physical means, such as microinjection (9 ), laser beams (10 ), and electroporation (11 ). However, none of these methods work with high efficiency on every cell type, whether freshly isolated cells or established cell lines. Some of these techniques, such as microinjection and laser-mediated transfection, require sophisticated apparatus and are technically difficult, whereas others, such as protoplast fusion, retroviral vectors, and liposome fusion, require time-consuming biochemical manipulations. Electroporation, which circumvents most of these problems, has emerged as an effective tool for the transfection of eukaryotic cells in suspensions (for a review,seeref. 12 ).
