The determination of the DNA sequence (genetic code) of chromosome 21 was the culmination of decades of work by laboratories throughout the world. The DNA sequence is the ultimate map of the chromosome. Determination of the DNA sequence allows, in principle, the precise identification of every gene contained on the chromosome. This information will be essential for understanding the genetic underpinnings of Down syndrome as well as for understanding many genetic disorders like Alzheimer's disease, Lou Gehrig's disease, and several forms of cancer attributable to genes on chromosome 21. Ultimately, the goal is to use this information to devise treatments to ameliorate the effects of Down syndrome and other conditions.

The mapping of the chromosome started from the cytogenetic representation of chromosome 21, which consists of 12 regions visible under the microscope, called brands. Since the chromosome contain 33.5 million base pairs of genetic code, one can see that the resolution of the cytogenetic map is quite poor. Improving the resolution of the map was an essential step in the ultimate sequencing of the chromosome. Several phases of mapping resolution were achieved, each building upon and adding to each other. In 1996, the chromosome 21 research community felt that the mapping had reached a sufficient level of refinement that sequencing could commence, and the Chromosome 21 sequencing consortium was officially launched. By May of 2000, the task had been completed.

Much can be learned from analysis of the DNA sequence itself. For example, one surprising finding is the number of genes contained on chromosome 21 is relatively low, at about 225. In addition, there are large regions of the chromosome that are particularly gene poor. This information is, however, jus the beginning of a true understanding of how the presence of an extra chromosome 21 leads to Down syndrome. To achieve this goal, we now need to know what contribution each gene makes to Down syndrome and understand how to counteract the deleterious effects of having an extra copy of each gene that contributes. How this information may be gained and what its implications are likely to be will be discussed.

For centuries, people with Down syndrome have been alluded to in art, literature and science. It wasn't until the late 19th century, however, that John Langdon Down, an English physician, published an accurate description of a person with Down syndrome. It was this scholarly work, published in 1866, which earned Down the recognition as the "father" of the syndrome. Although other people had previously recognized the characteristics of the syndrome, it was Down who described the condition as a distinct and separate entity.

Throughout the 20th century, advances in medicine and science enabled researchers to investigate the characteristics of people with Down syndrome. In 1959, the French physician, Jerome Lejeune, identified Down syndrome as a chromosomal anomaly. Instead of the usual 46 chromosomes present in each cell, Lejeune observed 47 in the cells of individuals with Down syndrome. It was later determined that an extra partial or complete 21st chromosome results in the characteristics associated with Down syndrome.

In June 2001, a study was published by the Center for Disease Control and Prevention indicating a stark racial disparity in the median life spans of people with Down syndrome. The study, based on an analysis of 34,000 multiple-cause mortality files from 1968 to 1997, indicated that the median age at death for white people with Down syndrome is 50 years, while it is 25 years for black people and 11 years for people of other races.