The international conference on Down Syndrome research will be held in the United States under the auspices of the National Down Syndrome Society. The meeting will review and document the current status of the entire field of Down Syndrome research and examine the directions that future research about this genetic disorder should take. The meeting will focus on five major topics: l) the structure of human chromosome 21, 2) the cause(s) and mechanisms of non.disjunction and prenatal detection, 3) the components of the Down Syndrome phenotype, 4) mental retardation, and 5) Alzheimer disease in Down Syndrome. There will also be two poster sessions for scientists to describe work on specific topics. Both the speakers and attendees will include Americans and representatives of countries throughout the world. The meeting will provide an opportunity for the participants to network with one another to share their knowledge, develop new collaborations, and facilitate the thrusting of Down Syndrome research forward in new directions. The conference will be convened under the direction of the National Down Syndrome Society Science Advisory Board.