Localization of specific mRNA and protein molecules within cells and tissues can provide important information on the effects of a drug within a biological test system and help elucidate mechanisms of drug-induced toxicity and organ dysfunction. The most widely used techniques for measuring the cellular and tissue distribution of mRNA and protein are in-situ hybridization (ISH) and immunocytochemistry (ICC), respectively. These can be applied alongside quantitative measurements to provide an integrated picture of gene expression. In some cases, for example when the gene expression of interest is confined to a small subset of cells within a tissue, histochemical techniques may be the preferred method of analysis.