In situ hybridization is the hybridization-mediated detection of specific nucleic acid sequences within structurally intact cells or tissues. As such it uniquely provides localization of nucleic acid superimposed on observable cellular and subcellular structural detail, allowing analysis unobtainable by other hybridization techniques. The technique is highly sensitive, particularly when the target nucleic acid is contained within a small percentage of a sample of cells. Innovations have increased the versatility of in situ hybridization which is now capable of specific detection of DNA, or RNA of sense or anti-sense polarity, application to samples prepared with a variety of fixation and embedding procedures, and analysis at the macroscopic, light microscopic, or electron microscopic level. These characteristics have made in situ hybridization a powerful and important means of analysis in a diversity of scientific fields.