Functional gene cassettes in development.

It has been noted in recent years that groups ofgenes are often used at multiple stages and multiple places of development. Comparison of the functions of each group of genes in different developmental context gave rise to the notion of "functional gene cassettes." These groups of genes can often be viewed as sets of tools; each group is suited to carry out a certain operation and may serve similar functions in several developmental processes. For example, neurogenic genes can be thought of as a set of tools for singling out cells from a group of equivalent cells. The affected cells could be neuroectodermal cells, myoblasts, or a number of other cell types encountered at various stages of development (see below). A corollary is that if one notices that one member of a group is used in any developmental process, then there is good reason to suspect that other members may also be used. This simple notion provides a rather useful way of attempting to identify genes that control a hitherto poorly characterized developmental process. In the following, we will illustrate this notion with examples of three groups of genes. All three groups have important functions in controlling early steps of Drosophila neurogenesis. Each group is also utilized in other developmental processes.