Chromosome 21 is one of the 23 pairs of human chromosomes. It has 48 million base pairs (the building material of DNA). This is about 1.5 percent of the total DNA in cells. Chromosome 21 is the smallest human autosome (non-sex chromosome).
In May 2000, researchers working on the Human Genome Project announced that the sequence of base pairs which make up this chromosome had been identified. Chromosome 21 was the second human chromosome to be completely sequenced. Chromosome 22 was the first.