Following the completion of the mouse and human genome sequences, a major challengeisthefunctionalcharacterizationofeverymammaliangeneandthedeciph- ing of their molecular interaction network. The mouse offers many advantages for the use of genetics to study human biology and disease, unmatched among other m- mals. Its development, body plan, physiology, behavior, and diseases have much in common, based on the fact that 99% of the human genes have a mouse ortholog. The investigation of gene function using mouse models is based on many years of tech- logical development. In the two decades...