The past 10 years have witnessed an increasing application of in situ hybridization techniques in biomedical research. This is mainly a result of the development of nonradioisotopic nucleic acid hybridization detection formats. Such methods allow much better exploitation of the spatial resolution provided by light microscopy than radioisotopic detection formats such as microautoradiography (1 , 2 ). Also the spectral resolution of light microscopy can be used to advantage with such methods, because they readily enable the simultaneous detection of multiple nucleic acid sequence targets on the basis of different colors (3 –5 ). In situ hybridization techniques are now accepted as powerful research tools in molecular cell biology and genetics (for reviews see refs. (6 –8 ).