One of the most informative ways to investigate the function of a gene is to determine its effect in vivo, by artificially introducing DNA into the cells of an organism. Such DNA is called a transgene (transferred gene), and such organisms are known as transgenic. Transgenes can be used to investigate gene function either by introducing a new gene, new regulatory regions, or a mutated version of an endogenous gene, or by inactivating an endogenous gene. A new or enhanced gene activity conferred by a transgene is known as a gain-offunction mutation, whereas a loss-of-function mutation results in reduced or abolished gene function.
Common transgenic animals, in order of complexity, include baker’s yeast (Saccharomyces cerevisiae), the nematode worm (Caenorhabditis elegans), the fruit fly (Drosophila melanogaster), and the mouse (Mus musculus). Evolutionary conservation means that their genes share a degree of homology with human genes, so that discoveries about function made with these organisms frequently apply to humans. While similar genetic manipulations can be performed in vitro using cell culture, transgenic organisms provide more physiologically relevant data, resulting from the interaction of gene products with all components of an intact organism. Transgenic organisms therefore provide models to explore gene function, gene control, and human diseases