Array experiments measuring large numbers of transcription or expression levels are another form of genome-wide analysis that have become widespread [14]. Although the effect sizes expected in these studies are large by comparison with disease association studies, the sample sizes are constrained by cost to be relatively small, so that both types of study encounter problems of statistical power (Table 1). Expression levels can be regarded as quantitative traits under genetic control, so that both kinds of large-scale exploration can occur in genome scans for loci influencing expression levels [15], or phenome scans demarking the influence of genetic pathways [16,17].
