Knowing the differences in gene expression levels among different tissues is of paramount interest in various areas of biomedical research. These include, but are by no means limited to, (i) comparisons of healthy and diseased tissue in order to understand malfunctions in regulation and identify genes essential for control, thus identifying target molecules for the development of novel therapeutics and (ii) the observation of changes in expression over time after addition of a drug to elucidate the mechanism of action of pharmaceuticals and predict their toxicology. Experiments in that realm furthermore help to understand the molecular basis of diseases, provide means for early diagnostics, and facilitate monitoring of therapy.