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.