Eye development in Drosophila involves the Notch signaling pathway at several consecutive steps. At first, Notch signaling is required for stable expression of the proneural gene atonal (ato), thereby maintaining neural potential of the cells. Second, in a process of lateral inhibition, Notch signaling is necessary to confine neural commitment to individual photoreceptor founder cells. Later on, the successive addition of cells to maturing ommatidia is under Notch control. In contrast to previous assumptions, the recessive Notch allele split (Nspl) involves specifically loss of the early proneural Notch activity in the eye, which is in agreement with bristle defects as well. As a result, fewer cells gain neural potential and fewer ommatidia are founded. Enhancement of this phenotype by the dominant mutation Enhancer of split [E(spl)D] happens within the remaining proneural cells, in which Ato expression is abolished. In line with genetic data, this process occurs primarily at the protein level due to altered protein-protein interactions between the aberrant E(spl)D and proneural proteins. Nspl is the first Notch mutation known to specifically affect Notch inductive processes during eye development.