Homeobox genes and the vertebrate head.
Several Drosophila genes important in the control of embryonic development contain a characteristic sequence of DNA, known as the homeobox. Homeobox sequences are also present in a family of vertebrate genes, which may therefore have regulatory roles during vertebrate embryogenesis. In this article, data concerning the spatial patterns of vertebrate homeobox gene expression are discussed in relation to recent descriptive and experimental analyses of head development. It is concluded that the patterns of gene expression are consistent with homeobox genes having roles in anteroposterior positional specification within the developing brain and possibly the neural crest. The data are not clearly consistent with these genes having direct roles in controlling the patterns of cranial segmentation, although further studies may reveal whether vertebrate segments are units of developmental specification.