Although DNA transmits genetic information through time, it basically has a passive role. Proteins encoded by DNA actually carry out the myriad cellular reactions that constitute "life." Now that the Human Genome Project has provided us with a catalog of tens of thousands of genes, we are left with the question: "What do proteins made by these genes actually do?"
Scientists have always looked to mutant organisms to provide clues about protein function. Now, specific mutants can be created at will by inserting an altered or non-functioning copy of a gene back into a living organism, then looking for changes in behavior or development. Since mice breed quickly and share about 99% of their genes with humans, they have become the animal model of choice for large-scale functional studies. However, doing a single transgenic experiment is several orders of magnitude more difficult than sequencing the gene itself. The real work of understanding the human genome still lies ahead.