Of the four "Drosophila" Pax6 orthologues, it is thought that the "eyeless" (ey) and "twin of eyeless" (toy) gene products share functional homology with the vertebrate canonical Pax6 isoform, while the "eyegone" (eyg) and "twin of eyegone" (toe) gene products share functional homology with the vertebrate Pax6(5a) isoform. "Eyeless" and "eyegone" were named for their respective mutant phenotypes. These paralogs also play a role in the development in the entire eye-antennal disc, and consequently in head formation. "toy" positively regulates "ey" expression.