The past few years have witnessed an extraordinary surge of interest in the microarray. The profusion of sequenced genes of uncertain function—from those of Arabidopsis thaliana to those of the most recent 'arrival', Caenorhabditis elegans — is daunting. The vast numbers of loci assayed in genome scans are testimony to the substantive challenge of the complex trait. The microarray promises to tackle these problems by providing insight into gene function (via expression studies) and the relevance of genetic loci to phenotypic traits, using a systematic global strategy instead of a piecemeal one.

While intimately familiar with the practicalities of making a Southern blot, the forerunner of the microarray, many researchers lack a 'nuts–and–bolts' appreciation of the different types of microarray and their manufacture and processing. A detailed and thorough understanding of the options available improves the odds that one's choice will be vindicated; an appreciation of the biological context of genetic (and genomic) questions can also be critical to the success of microarray analysis. The articles in this special supplement of Nature Genetics have been selected with these issues in mind. The limitations and the ultimate utility of the microarray remain to be determined, however, as pointed out in the Foreword by Francis Collins of the National Human Genome Research Institute (NHGRI) — the generous sponsor of this supplement. It is hoped that in addition to serving as a practical resource, the contents of this supplement (which will be publicly available online) will provide a glimpse of things to come.Box 1