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 coli genome - 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.