Genetic linkage studies of a large family with DS established the location of a DS gene on chromosome 2. Although a genetic cause of DS has long been accepted, these studies were the first to show a statistically significant linkage. In addition, cytogenetic results of individuals with DS have shown, in rare cases, abnormalities that suggest the contribution of more than 1 gene. Deletions on chromosomes 4 and 8 and an extra marker chromosome thought to be derived from chromosome 22 have been documented in individuals with D