It is peculiar that in situ hybridization (ISH), a technique with many similarities to immunohistochemistry (IHC), has not enjoyed the phenomenal growth in both basic research and clinical applications as has its sister technique IHC. Since the late 1970s, when immunoperoxidase techniques began to be applied to routine diagnostic material and to numerous research applications, there has been a natural evolution of the IHC procedure. Namely, only a few primary antibodies were available commercially at the onset, and only one indirect and the peroxidase-antiperoxidase

(PAP) technique detection systems were in place. With the advent of avidin-biotin detection systems and monoclonal antibodies, and a viable commercial market, extraordinary growth of the procedure’s applications in clinical research and diagnostic pathology occurred during the subsequent two decades. Today, IHC is automated and widely used for research purposes and, to a large extent, has become a routine diagnostic “special stain” in most clinical laboratories.