An international consortium of laboratories has published an almost-complete sequence for human chromosome 21 [
Hattori, M. et al (2000) Nature 405, 311-319; see also commentary by Reeves [Nature (2000) 405, 283-284]. This is the chromosome associated with Down syndrome, the most common inherited cause of mental retardation: the syndrome occurs in individuals who have three rather than the normal two copies of chromosome 21. A surprise emerging from the complete sequence of the chromosome is the very small number of genes it contains: only about 225 compared with more than twice that number on the similarly-sized long arm of chromosome 22. This small number may explain why possession of an extra copy of the chromosome is not usually lethal either very early in development or soon after birth, as it is for every other chromosome. The relatively small number of genes on chromosome 21 has increased hopes that knowledge of its genetic content might help in achieving an understanding of which genes contribute most to the characteristics of Down syndrome. Eventually this knowledge might lead to treatments to alleviate some of these problems.