Immunohistochemistry involves the binding of an antibody to a cellular or tissue antigen of interest and then visualization of the bound product by a detection system. With the ever-increasing number of antibodies against cellular epitopes, immunohistochemistry is an extremely useful diagnostic tool as well as a means to guide specific therapies that target a particular antigen on neoplastic cells. As many antibodies are effective in fixed paraffin-embedded tissues, large retrospective studies of protein expression in a variety of human cancers can easily be performed.