the use of genetic technology to determine the function of newly discovered genes by determining their role in one or more model organisms. functional genomics uses as its starting point the isolated gene whose function is to be determined, and then selects a model organism in which a homolog of that gene exists. this model organism can be as simple as a yeast cell or as complex as a nematode worm, fruitfly, or even a mouse. the gene is selectively inactivated or "knocked out" using a variety of genetic techniques, and the effect of its selective deletion on that organism is determined. by knocking out a gene in this way, its contribution to the function of the organism (and, by implication, its function in man), can be determined. functional genomics has proven particularly useful as a means of validating or testing novel therapeutic targets. in another approach, a whole set of genes may be systematically inactivated and the effect of this on a particular cellular function examined. here, a new gene and its function are identified simultaneously.