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).

There are two copies of chromosome 21 in most people. Anyone with three copies of chromosome 21 has Down syndrome, also known as trisomy 21.