Sequence of human chromosome 21

Chromosome 21 may have as few as 225 active genes in its 33.8 Mb of DNA, according to the sequence published electronically by Nature on May 8 (and published in print on May 18 as Hattori et al. Nature 2000, 405:311-319). This is roughly half the gene density of the only other completely sequenced chromosome arm, that of chromosome 22, and it includes a 7 Mb stretch - larger than the Escherichia coligenome - that contains only one gene. Extrapolating from the average gene density of chromosomes 21 and 22 gives a total number of human genes that is closer to 40,000 than the estimated 70-140,000. The low density of genes on chromosome 21 may explain why children with Down's syndrome, who are trisomic for chromosome 21, can survive.