Dr. Stephen J. O'Brien is head of the Laboratory of Genomic Diversity at the National Cancer Institutes, National Institutes of Health. Dr. O'Brien is internationally recognized for his research contributions in human and animal genetics, evolutionary biology, retrovirology, and species conservation. In collaboration with his students, fellows, and colleagues he has researched areas as diverse as mapping the genome of the cat, to the discovery of CCR5-32, the first human gene shown to block infection by HIV among its carriers. Dr. O'Brien is the author or co-author of over 500 scientific articles that have appeared widely in" National Geographic, Scientific American, Nature "and "Science".