End of the line? Tramtrack and cell fate determination in Drosophila.

Cell differentiation reflects the balance of two opposing influences, pathways which confer specialized properties on specific cells or groups of cells, and antagonising mechanisms which modulate responsiveness to such differentiative cues. It appears that the zinc finger protein Tramtrack (Ttk) fulfils the latter function in the CNS and PNS of Drosophila. Ttk seems to be able to inhibit neural development by down-regulating competence to respond to neuralising signals. We speculate, however, that restriction of neural competence is merely one example of a more general ability of Ttk to influence differentiation and that, given its widespread expression profile, Ttk might be implicated in a number of differentiative events.