Evolutionarily conserved sequences on human chromosome 21

Comparison of human sequences with the DNA of other mammals is an excellent means of identifying functional elements in the human genome. Here we desc ribe the utility of high-density oligonucleotide arrays as a rapid approach for comparing human sequences with the DNA of multiple species whose seque nces are not presently available. High-density arrays representing similar to 22.5 Mb of nonrepetitive human chromosome 21 sequence were synthesized a nd then hybridized with mouse and dog DNA to identify sequences conserved b etween humans and mice (human-mouse elements) and between humans and dogs ( human-dog elements). Our data show that sequence comparison of multiple spe cies provides a powerful empiric method for identifying actively conserved elements in the human genome. A large fraction of these evolutionarily cons erved elements are present in regions on chromosome 21 that do not encode k nown genes.