Recent progress in techniques for the transfer of genes into cultured mammalian cells has made possible the isolation of various interesting genes from other mammalian cells, and also the study of the function and the regulation of gene expression of mammalian genes in vivo. Various transfection techniques have been developed: incubation of recipient cells with genes in the presence of diethylaminoethyldextran (see Chapter 3 , this vol.; ref. 1 ); incubation of the cells with metaphase chromosomes and poly-L-ornithine (2 ); and incubation of the cells with coprecipitates of calcium phosphate and genetic material, such as DNA (Chapter 2 ; refs. 3 ,4 ), metaphase chromosomes (Chapter 10 ; ref. 5 ), and λ phage particles (6 ,7 ). Other methods for gene transfer have also been developed: direct injection of DNA into the nuclei of recipient cells (8 ,9 ); use of viral vectors (Chapter 11 –Chapter 15 ; for review, see also ref. 10 ); fusion of the recipient cells with bacterial spheroplasts (11 ) or liposomes (10 –14 ); and fusion of cells, mediated by HVJ (Sendai virus), with liposomes (15 –17 ) or reconstituted erythrocyte membrane vesicles (18 ). The cell membranes of recipient cells have also been rendered permeable to genetic material by electric pulses (Chapter 5 ; ref. 19 )Chapter 11 Chapter 13 15 ; for review, see also ref. 10 ); fusion of the recipient cells with bacterial spheroplasts (11 ) or liposomes (10 –14 ); and fusion of cells, mediated by HVJ (Sendai virus), with liposomes (15 –17 ) or reconstituted erythrocyte membrane vesicles (18 ). The cell membranes of recipient cells have also been rendered permeable to genetic material by electric pulses (Chapter 5 ; ref. 19 )