We analyzed the expression and function of eyeless (ey) and twin of eyeless 
(toy) in the embryonic central nervous system (CNS) of Drosophila. Both genes 
are differentially expressed in specific neuronal subsets (but not in glia) in 
every CNS neuromere, and in the brain, specific cell populations co-expressing 
both proteins define a longitudinal domain which is intercalated between broad 
exclusive expression domains of ey and toy. Studies of genetic null alleles and 
dsRNA interference did not reveal any gross neuroanatomical effects of ey, toy, 
or ey/toy elimination in the embryonic CNS. In contrast, targeted misexpression 
of ey, but not of toy, resulted in profound axonal abnormalities in the 
embryonic ventral nerve cord and brain.