Sensitive techniques developed to detect biologically relevant proteins at the mRNA and protein levels have been major research tools in basic and applied biomedical research (1 -6 ). The combination of these two techniques has been particularly valuable when the biological protein being studied is a secreted cell product that has the ability to bind to components of the extracellular matrix or to receptors on target cells within the same tissue (4 ,5 ,7 -9 ). The best example of this is the study of cytokine production in human tissue sections, where standard immunohistochemical labeling techniques will detect cytokine production, secreted cytokine, and cytokine bound to receptors on target cells (4 ,5 ,9 ). If it is important to determine which of the cellular components of the tissue under study is producing the biological protein, the combination ofin situhybridization to detect mRNA and immunohistochemical labeling to detect the protein offers the ability to distinguish between the cells responsible for production of the protein and its target cells.
