Examples of the results in Zhang et al.  (reproduced with permission). Each row of the image represents a gene and each column a tissue (labeled at the bottom), with the color indicating the ratio of expression levels. Clusters of genes coordinately regulated in certain tissues correlate with the biological functions indicated on the right. See the complete article  for a full version of this figure.
Holmes and Brown Journal of Biology 2004 3:20 doi:10.1186/jbiol19