Nature 405, 311— 319; 2000.

The name of one author, Y. Groner, was inadvertantly omitted from the author list but is included above. He is affiliated with the Department of Molecular Genetics and Human Genome Center, The Weizmann Institute of Science, Rehovot, Israel. Also, in the second paragraph of the section headed ‘Comparison of chromosome 21 with other autosomes’, the second sequence should read ‘For example, a 100-kb region of a contig (AP001656) on 21p is shared with chromosomes 4, 7, 20 and 22.’