Non-isotopic whole-mount in situ hybridization of mRNA is a novel technique that has greatlgfacllitated the precise three. dimensional localization of transcripts from genes whose expression is important during development. This methodology has recently been applied to the study of the mouse embryo and offers particular advantages over conventional procedures. sections that are often distorted or incomplete. What is more, the opportunity for inspecting histological detail is not lost, since embryos can be sectioned after staining.

Whole-mount procedures also save time and labour compared with autoradiography of sections. Large numbers of intact embryos, of the same or different developmental stage or genotype, can be analysed simultaneously. Therefore, gene expression can be charted in a population of embryos rather than in multiple sections from two or three embryos, making it easier to ascertain the reproducibility of results. It is also easier to identify very restricted sites of gene expression by examining intact embryos than by looking at large numbers of serial sections. These considerations are particularly relevant when mapping the expression pattern of novel genes through the various stages of development.