Transcription factor both necessary and sufficient for proper development of most cell types of the embryonic and adult peripheral nervous system (PNS). Essential component of the proneural Notch signaling pathway required for proper sensory organ precursor (SOP) differentiation. Correct expression requires expression of scalloped (sd). Repression of rough (ro) in R8 photoreceptor is an essential mechanism of R8 cell fate determination. Almost exclusively restricted to precursors and early differentiating cells of the embryonic PNS. Expressed in the R8 photoreceptor during third instar eye development beginning within the morphogenetic furrow of the eye imaginal disk. Mutants abolish the further up-regulation and maintenance of proneural gene expression in the SOPs.