Mutations at the X-linked disconnected locus of D. melanogaster lead to the failure of adult photoreceptor axons to innervate their target cells in the developing optic lobes of the third instar larva, resulting in flies that have rudimentary optic ganglia. The cascade of epigenetic events leading to the adult disconnected phenotype is caused by the misrouting of a larval pioneer nerve, Bolwig's nerve, during embryonic development. In the disconnected mutant this nerve fails to recognize and establish stable connections with its correct synaptic partners. In addition, disconnected affects both the proper aggregation and the movement of the Bolwig neurons to their final location in the embryo. Finally, similar but more subtle defects can be found in a subset of other peripheral neurons in the thoracic and abdominal segments. The different aspects of the phenotype suggest that the disconnected gene plays a role in neuronal cell recognition.