Either a direct or an indirect assay principle can be chosen, and one can select from a large number of detection and labeling formats. Under certain conditions, one has to adapt a specific staining technique according to the needs of the tissue and the molecules under study. The principles of immunohistological staining can be also applied for the detection of target molecules other than antigens supposed that selective probes are available. With the experience of immunocytochemistry, a number of non-immunological affinity detection principles have become developped. For example, molecular affinity bindings of lectins and nucleic acids (in situ hybridization, FISH etc.) evolved from immunohistological detection principles. The detection formats include enzymatic and nonenzymatic labels, avidin-biotin principles, antibody-protein A bindings and other types of ligand bindings.