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The Biology of Down Syndrome

Down syndrome is unique because of its preeminent role in shedding light on the biology of mental retardation. Since this disorder was first described in 1866 by John Langdon Down, the study of Down syndrome has contributed important biological knowledge and insight into normal and abnormal human development. Furthermore, Down syndrome research has provided useful information to such disparate scientific fields as oncology, immunology, biochemistry, and the behavioral sciences. The discovery by Professor Lejeune, in 1959, of an extra number 21 chromosome in the cells of individuals with Down syndrome, along with the simultaneous discovery of sex chromosome aneuploidy, firmly established cytogenetics as a bona fide medical discipline.' Although much has been learned about Down syndrome in the past 120 years, many fundamental questions remain unanswered.2 These include, but are not limited to, questions regarding the basis for the relationship between maternal age and nondisjunction; the mechanisms resulting in predispositions toward leukemias; immunologic abnormalities inherent in Down syndrome; and the genes responsible for the clinical phenotype. With the advent of molecular biology, new research methods may now be applied to these previously elusive questions.