Genomics aims to provide biologists with the equivalent of chemistrys Periodic Table 1 an inventory of all genes used to assemble a living creature, together with an insightful system for classifying these building blocks. A short decade ago, the task of enumeration alone appeared to many to be a quixotic quest. Whereas chemical matter is composed of a mere hundred or so elements, organismal parts lists are hugerunning into the thousands for bacteria and hundreds of thousands for mammals. Genomic mapping and sequencing, however, has steadily extended its dominion: it has domesticated the Megabase and will tame the Gigabase in the not-too-distant future.
The next great challenge is to discern the underlying order. The Periodic Table summarized chemical propensities in its rows and columns, and thereby foreshadowed the secrets of subatomic struc-ture. Understanding biological systems with 100,000 genes will sim-ilarly require organizing the parts by their properties. The Biological Periodic Table will not be two-dimensional, but will reflect similarities at diverse levels: primary DNA sequence in cod-ing and regulatory regions; polymorphic variation within a species or subgroup; time and place of expression of RNAs during develop-ment, physiological response and disease; and subcellular localiza-tion and intermolecular interaction of protein products. The traditional gene-by-gene approach will not suffice to meet the sheer magnitude of the problem. It will be necessary to take global views of biological processes: simultaneous readouts of all components.
Arrays offer the first great hope for such global views by pro-viding a systematic way to survey DNA and RNA variation. They seem likely to become a standard tool of both molecular biology research and clinical diagnostics. These prospects have attracted great interest and investment from both the public and private sectors. The reviews in this supplement describe important issues in this fast-moving area.