Neuron subtypes are specified through diversity: each subtype gets its own transcription factor. But in the lateral glia of Drosophila, specification of all glial cell types is controlled by a single fate-determining gene called Glide/Gcm (gcm). How can just one gene make many glial cell types? Rossana De Iaco, Angela Giangrande (National Center for Scientific Research in Strasbourg, France), and colleagues provide the solution for one type of glial cell. They show that although low expression of the gcm gene make cells competent to become glia, a boost in expression level regulates the type of glia that cells will become. Figure 1 Gcm specifies glia (red), but the glial subset NGB1-1A requires higher Gcm levels (top). Glia fail to form in mutant embryos lacking gcm. In mutants lacking a patterning protein called Huckebein (Hkb), one type of glia—the product of one specific neuroglioblast lineage—fails to form. By following this lineage under different conditions of gcm and hkb expression, the researchers found that Hkb binds to the Gcm protein to up-regulate gcm expression. Hkb thus provides a molecular link between gcm's general role in specifying all glia and its subdividing role in specifying different glial cell types. Although Hkb itself can be seen as a lineage-specific factor, the fact that it acts by altering levels of a cell fate determinant is unusual. Giangrande speculates that such quantitative regulation of cell types might operate in animals besides flies. Reference: De Iaco, R., et al. 2005. EMBO J. doi:.10.1038/sj.emboj.7600907 [PubMed] [Cross Ref]