Identification of gene expression modules in cardiac disease. Highly expressed genes in the heart were sorted using hierarchical clustering (dendrogram shown on left) based on the similarity in their expression pattern over different types of cardiac disease. These relationships are visualized on a correlation matrix (center), in which each pixel represents the correlation between a pair of genes, and each row represents the relationship between one gene and all others. The color scale (right) represents the degree of correlation, ranging from strongly positive (red) to uncorrelated (black) to strongly negative (green). Boxes along the diagonal of the matrix correspond to modules formed by chopping the dendrogram at the level indicated by the dotted line (determined by maximizing the number of genes in each module associated with the same gene ontology term; see methods). Boxes shown in bold contain genes with particular relevance to cardiac disease, and are shown in detail in figure 3. The bold portion of the dendrogram is also shown in Figure 3.