Fig. 10. The epistatic relationships in the salivary glands and in the peripheral nervous system. In the salivary glands, Scr activates fkh and sage. fkh is necessary for the initiation (I) of sens expression. sage, together with da, is necessary for the maintenance (M) of sens expression. In turn, sens is necessary to maintain the expression of sage in the salivary glands via a positive feedback loop. sens represses reaper and hid and prevents cell death. By contrast, in the peripheral nervous system, achaete-scute complex genes (AS-C) and da are needed together to initiate and maintain sens expression (Nolo et al., 2000). By a feedback loop similar to that in the salivary glands, sens amplifies the expression of the AS-C genes. Downstream from sens, it represses E(spl) transcription and is necessary for neuronal cell fate determination.