When describing a dog, Mark Twain once wrote, “The dog is a gentleman; I hope to go to his heaven, not man's.” Twain's simple comment reflects both our admiration for the loyalty, integrity, and devotion we have come to expect from our closest companions, and our desire to keep them ever at our side. In the last few years, as summarized here, the canine genome project has worked tirelessly to develop resources and paradigms that will lead to both the improvement of animal and human health and an understanding of the genetics that regulates variation between breeds. A great deal remains to be learned. We still don't know why Great Danes are big and Pekingese are small, or why herding dogs herd and pointing dogs point. But in another sense, we have succeeded beyond our wildest dreams, as the dog is now a viable system in which to tackle problems relating to the genetics of complex traits.
We thank Liz McNeil, Ed Giniger, Cheryl Maslen, K. Gordon Lark, and Kerstin Lindblad-Toh for their careful reading of the manuscript and useful comments. This work was supported in part by funds from the National Human Genome Research Institute Intramural Program. Finally, we gratefully acknowledge the many dog owners and breeders who continue to support our work by providing samples, pedigrees, and clinical information on their pets.