08 May 2000

The DNA sequence of human chromosome 21 will be published electronically today in Nature (and in print on 18 May). This achievement, by an international consortium of 62 scientists in Japan, Germany, France, Switzerland, the USA and Britain, reveals that chromosome 21 may have as few as 225 active genes in its 33.8 megabases of DNA-just half as many as the 545 reported in the 33.4 Mb of chromosome 22, the first human chromosome to be sequenced and published late last year (see Dunham I et al., 1999).

Large expanses of chromosome 21 appear to do nothing. For example, one 7-megabase stretch, larger than the entire genome of the bacterium Escherichia coli, contains precisely one gene. If the averaged gene density of both chromosomes 21 and 22 are representative, the total number of human genes may be closer to 40,000 than to previous estimates of between 70,000 and 140,000. This paucity of genes is itself interesting, as it may explain why people with an extra copy (or "trisomy") of chomosome 21 survive to adulthood despite a spectrum of physiological disorders known as Down's syndrome. Analysis of the genes on chromosome 21 may permit a deeper understanding of Down's syndrome and its complications, as well as a range of other linked genetic disorders such as Alzheimer's disease and certain forms of cancer.—Hakon Heimer