The clinical, cytogenetic, and molecular studies of an individual are presented here for the purpose of further characterizing what regions of chromosome 21q are essential for expression of the typical Down syndrome phenotype. This individual had a de novo, unbalanced translocation chromosome interpreted as: 45,XX,t(18;dup[21q]). Physical examination revealed mild manifestations, but not the typical phenotype of Down syndrome. The patient was studied using 20 single copy probes known to map to the 21q region. DNA polymorphism and dosage analyses showed triplication of loci D21S13 through D21S58 involving 21q11 to 21q22.1, and possibly involving the 21q22.2 region. Her clinical presentation, which did not include the classical findings of Down syndrome, suggests that regions distal to 21q22.1, when present in triplicate dose, account for the major manifestations of the classical phenotype. However, duplication of the area between the centromere and 21q22.1 may contribute to some of the other abnormalities in Down syndrome.