Gene targeting: an historical perspective

Our entry into what was going to become the field of gene targeting began in 1977. I was experimenting with the use of extremely small glass needles to inject DNA directly into the nuclei of living cells. The needles were controlled by hydraulically driven micro-manipulators and were directed into nuclei with the aid of a microscope. Using this experimental paradigm, I asked myself whether I could introduce a functional gene into cells by injecting the DNA directly into their nuclei. This procedure turned out to be extremely efficient. One in three cells received the DNA in functional form and went on to divide and stably pass the gene onto its daughter cells (Capecchi, 1980). The high efficiency of micro-injection meant that it was now practical to use this technology to generate transgenic mice by the injection of DNA into one-cell zygotes. The embryos were then allowed to come to term by surgical transfer of the zygote to foster mothers. Indeed, this has become a cottage industry in many laboratories throughout the world (Gordon et al, 1980; Costantini and Lacy, 1981; Brinster et al, 1981; Wagner, E.F. et al, 1981; Wagner, T.C. et al, 1981) . However, generation of transgenic animals in this way involves the introduction of exogenous DNA segments at unpredictable locations in the recipient genome, and not targeted genetic alterations at defined sites.