The glial cells missing (gem) gene in Drosophila encodes a novel nucle
ar protein that is transiently expressed early in the development of n
early all glia, In loss-of-function gem mutant alleles, nearly all gli
a fail to differentiate, and, where we can follow them in the PNS, are
transformed into neurons. In gain-of-function gem conditions using tr
ansgenic constructs that drive ectopic gem expression, many presumptiv
e neurons are transformed into glia. Thus, gem appears to function as
a binary genetic switch for glia versus neurons. In the presence of ge
m protein, presumptive neurons become glia, while in its absence, pres
umptive glia become neurons.