Down syndrome (DS) is a major cause of mental retardation and heart disease. 
Although it is usually caused by the presence of an extra chromosome 21, a 
subset of the diagnostic features may be caused by the presence of only band 
21q22. We now present evidence that significantly narrows the chromosomal region 
responsible for several of the phenotypic features of DS. We report a molecular 
and cytogenetic analysis of a three-generation family containing four 
individuals with clinical DS as manifested by the characteristic facial 
appearance, endocardial cushion defect, mental retardation, and probably 
dermatoglyphic changes. Autoradiograms of quantitative Southern blots of DNAs 
from two affected sisters, their carrier father, and a normal control were 
analyzed after hybridization with two to six unique DNA sequences regionally 
mapped on chromosome 21. These include cDNA probes for the genes for 
CuZn-superoxide dismutase (SOD1) mapping in 21q22.1 and for the amyloid 
precursor protein (APP) mapping in 21q11.2-21.05, in addition to six probes for 
single-copy sequences: D21S46 in 21q11.2-21.05, D21S47 and SF57 in 21q22.1-22.3, 
and D21S39, D21S42, and D21S43 in 21q22.3. All sequences located in 21q22.3 were 
present in three copies in the affected individuals, whereas those located 
proximal to this region were present in only two copies. In the carrier father, 
all DNA sequences were present in only two copies. Cytogenetic analysis of 
affected individuals employing R and G banding of prometaphase preparations 
combined with in situ hybridization revealed a translocation of the region from 
very distal 21q22.1 to 21qter to chromosome 4q. Except for a possible phenotypic 
contribution from the deletion of chromosome band 4q35, these data provide a 
molecular definition of the minimal region of chromosome 21 which, when 
duplicated, generates the facial features, heart defect, a component of the 
mental retardation, and probably several of the dermatoglyphic changes of DS. 
This region may include parts of bands 21q22.2 and 21q22.3, but it must exclude 
the genes S0D1 and APP and most of band 21q22.1, specifically the region defined 
by S0D1, SF57 and D21S47.