The human genome project reached a major milestone earlier this year, with the completion of a rough draft of the sequence (Pennisi 2000). An accessible catalog of all human genes could greatly accelerate the pace of breakthroughs in medical research by allowing global analyses of gene expression changes in a variety of developmental and pathological states. However, it has never been clear whether these types of analyses could be efficiently performed, or whether significant sets of important data would emerge from the study of large sets of genes. Many diseases arise from single gene mutations in which the corresponding protein products are known. Would knowledge of global gene expression changes add to our existing understanding of these monogenic disorders or lead to the development of treatments? A paper in this issue provides a test of these questions...

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