those genes represent. An alternative approach, pioneered by Gene Set Enrichment Analysis (GSEA) (Mootha et al., 2003; Subramanian et al., 2005), focuses on coordinated differential expression of annotated groups of genes, or gene sets, and produces results that can more easily be interpreted in terms of the relevant biological processes. Since its introduction, the use of GSEA has become widespread and has motivated the development of many similar approaches (reviewed in Huang et al., 2009a) and even novel statistical methods based on groups of variables (Efron, 2010; Good, 2011). Over the last decade GSEA has proven a very successful approach in many fields of biomedical research and has become an essential part of the genomic analysis toolbox.
