The introduction of recombinant DNA into mouse embryos has proved to be a useful technique in addressing a number of important developmental questions, e.g. the identification of DNA sequences controlling tissue-specific gene expression and the role of genes in growth control (for reviews see Palmiter & Brinster, 1985; Wagner & Stewart, 1986). Currently the favoured method for gene transfer into mice is by DNA injection into a pronucleus of the fertilized egg. However an alternative method is to exploit retroviruses as vectors for introducing genes either directly into embryos or into embryonal carcinoma (EC) or embryonic stem (ES) cells which can then be used to form chimaeric mice (Bradley, Evans, Kaufman & Robertson, 1984). There are a number of advantages to using retroviral vectors, the major one being that they can infect a wide variety of cells at an efficiency approaching 100 %.

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