**7. The multifunctional protein and its relation to cellular compartments**

A single protein can have different functions in a cell and these functions concern the compartment where they are located. One of the best documented examples is that transglutaminase 2 (TG2) may act as a transglutaminase, G-protein kinase, protein disulfide isomerase or as an adapter protein. These multiple biochemical activities are involved in a wide variety of cellular processes such as differentiation, cell death, inflammation, cell migration and others. The specific microhabitats and subcellular compartments of location of the plasma membrane, cytoplasm, nucleus, mitochondria, or extracellular space are important in the development of different biochemical activities by the same protein structure (Park et al., 2010).

Thus, in our search for a drug target, e.g., cancer, we must always know the location of a given protein in the cell and be aware of how this cell places these proteins in different micro environments, and that more often than not these different functions may occur simultaneously. So part of the strategy to find the correct pharmacological targets is the previous understanding of the structure and the establishment of subcellular microenvironments inside the cells and the better knowledge of the complex and dynamic subcellular compartmentalization that will be further explained.

Therefore, immunohistochemistry provides information that cannot be obtained in any other way, which is the relationship between the pathological state and the dimension of the altered compartment (Oliver and Jamur, 2009; Dabbs, 2010), of great relevance to the establishment of the cancer diagnosis, as we shall see soon.
