Serial Analysis of Gene Expression (SAGE) is a sequence-based genomics tool that features comprehensive gene discovery and quantitative gene expression capabilities. As an 'open' system, SAGE can reveal which genes are expressed and their level of expression rather than merely quantifying the expression level of a predetermined, and presently incomplete, set of genes as carried out by 'closed' system gene expression profiling platforms such as microarrays. These distinguishing attributes enable SAGE to be used as a primary discovery engine that can characterize human disease at the molecular level while illuminating potential targets and markers for therapeutic and diagnostic development, respectively.