In situ hybridization to mRNA: from black art to guiding light

In situ hybridization to mRNA in embryo sections or wholemount embryos is o ne of the most powerful analytical tools available to the molecular develop mental biologist. For many workers, this procedure provides the first insig hts into the function of newly isolated genes, allowing the formulation of hypotheses and setting the course for further research. This paper presents a personal historical perspective of the development of in situ hybridizat ion, looks at the theory and practice of the technique, summarizes the curr ent state of the art, and speculates on possible directions for the future as a tool in functional genomics.