Immunocytochemistry, together with biochemical procedures, has ensured rapid and accurate identification of tissue components. This has led to a better appreciation of cellular events, particularly storage and secretion of products. However, such techniques have certain drawbacks, in particular the impossibility of monitoring the intracellular processes concerned with protein synthesis. Thus, there was a need for a method that would provide more detailed information about the functional morphology and gene expression in tissues at cellular level. Advances in molecular biology allowed the development of in situ hybridization, a procedure that localizes specific nucleotide sequences (DNA or RNA) in tissue preparations using labeled complementary probes (DNA or RNA).