Seeing is believing with GloFish. They are absolutely stunning!" The preceding is some of the marketing material you'd read if you visited the GloFish website (GloFish, 2008). Beauty may be in the eye of the beholder, but nearly everyone would agree that these first—and, so far, only—transgenic animals made available to the general public in the United States (except in California, pending a formal review of their potential effect on the environment) are a worthy conversation piece. A transgenic, or genetically modified, organism is one that has been altered through recombinant DNA technology, which involves either the combining of DNA from different genomes or the insertion of foreign DNA into a genome. GloFish (Figure 1) are a type of transgenic zebrafish (Danio rerio) that have been modified through the insertion of a green fluorescent protein (gfp) gene. Not all GloFish are green, however. Rather, there are several gfp gene constructs, each encoding a different colored phenotype, from fluorescent yellow to fluorescent red.

Currently, GloFish are the only recombinant-DNA animal that has been approved for human "use" by the U.S. Food and Drug Administration. Their approval has raised important questions about whether, and to what extent, genetically modified animals should be made available to consumers. But how were scientists able to create these engineered organisms in the first place? Like so many genetic technologies used today, recombinant DNA technology had its origins in the late 1960s and early 1970s. By the 1960s, scientists had already learned that cells repair DNA breaks by reuniting, or recombining, the broken pieces. Thus, it was only a matter of time before researchers identified the raw biological ingredients necessary for recombination, figured out how those ingredients functioned together, and then tried to govern the recombining process themselves.