Expression of homeo box genes during mouse development: a review.

The aim of this review is to summarize briefly w^hat is presently known about the spatial and temporal patterns of homeo box gene expression during mouse development. Since information about mouse homeo box genes is accumulating very rapidly, this review does not offer a complete compendium of facts about topics such as gene sequence and organization. Instead, we have tried to assemble rather fragmentary published and unpublished data about expression into a tentative framework which may provide some clues as to the function of these genes. To simplify our analysis, we have concentrated upon three different stages of mouse development; early presomite gastrula (7-7.75 days post coitum*), early somite neurula (8-8.5 days p.c), and a stage about twothirds through gestation (12.5 days p.c.) when most of the organ systems have been established but are still undergoing morphogenesis. Detailed accounts of the anatomy of these embryos and the overall process of mouse development can be found in Snell and Stevens (1966), Theiler (1972), Rugh (1968), and Hogan et al. (1985, 1986). The homeo box is a DNA sequence of about 180 bp, originally identified within the coding region of several Drosophila genes controlling embryonic development (for review, see Gehring 1987). It is generally accepted that the protein products of Drosophila homeo box genes act as sequence-specific DNA binding proteins, regulating gene expression. This suggestion is consistent with the nuclear localization of several Drosophila homeo box gene products (White and Wilcox 1984; Beachy et al. 1985; Carroll and Scott 1985; Di Nardo et al. 1985), with their reported in vitro DNA binding properties (Desplan et al. 1985), and with their predicted protein structure (Shepherd et al. 1984; Laughon et al. 1985). Drosophila homeo box genes constitute a rather variable gene family. DNA sequence comparisons, primarily of the homeo box region itself, indicate that classes of genes exist within which the genes share a more recent evolutionary origin than that shared by the homeo box gene family as a whole. The most extensive class, for which Antennapedia (Antp) is the prototype, contains several of the Drosophila homeotic genes (including Antp, Sex combs reduced, Deformed, and infraabdo-