Down syndrome, as a phenotypic result of trisomy 21, is a complex condition
with a set of over 30 phenotypic features, wh ich manifest themselves with
varying frequencies among affected individuals. The importance for molecul
ar medicine of understanding the molecular mechanisms underlying Down syndr
ome becomes fully appreciated when a striking feature of Down syndrome is t
aken into account: that the overdose of otherwise perfectly normal genes ca
uses disorders of human health, indistinguishable from major public health
problems of the general population, such as mandatory early onset Alzheimer
's degeneration, increased risk of leukemia, and protection from cancer of
solid tissues. The DNA sequence of human chromosome 21 is, at the moment, t
he most complete piece of DNA sequence known in the whole of human genome.
The challenge for the future is an integrated, multidisciplinary approach t
o the molecular biology of chromosome 21 genes, in conjunction with the res
earch into the variation in their genotype, expression, and function in the
normal population, in Down syndrome individuals with well-characterized ph
enotypic traits, and in euploid patients suffering from diseases associated
with phenotypic components of Down syndrome: mental retardation, developme
ntal defects, hematological and solid tissue malignancies, and Alzheimer's
disease.