The Drosophila extramacrochaetae (emc) locus participates in sensory organ patterning by antagonizing, in a mechanistically unknown way, the neurogenic activity of the achaete-scute complex (AS-C). Our cloning of emc DNA and molecular mapping of emc mutations have identified a transcription unit as the most likely candidate for the emc function. It encodes a protein that has a dimerizing helix-loop-helix (HLH) motif but lacks a basic region presumably important for DNA binding. AS-C and other proneural proteins have both domains. We propose that the emc product antagonizes neurogenesis by sequestering proneural proteins in complexes inefficient for DNA interaction. These and other findings suggest the existence of a network of synergistic and antagonistic interactions, mediated by HLH proteins, that participates in the establishment of the neural fate.