Advances in our understanding of biological mechanisms have frequently been associated with the development of new techniques. In situ hybridization is a classic case of just such an advance. The technique effectively combines histochemistry with molecular biology and enables the rapid analysis of the distribution of RNA, or DNA, in the tissues. The information gained from this has caused something of a revolution in our understanding of developmental biology, since a fundamental aspect of development is the spatial and temporal expression of genes. In addition the technique has found...