The two Pax6 gene homologs eyeless and twin of eyeless play decisive early roles 
in Drosophila eye development. Strong mutants of twin of eyeless or of eyeless 
are headless, which suggests that they are required for the development of all 
structures derived from eye-antennal discs. The activity of these genes is 
crucial at the very beginning of eye-antennal development in the primordia of 
eye-antennal discs when eyeless is first activated by the twin of eyeless gene 
product. This activation does not strictly depend on the Twin of eyeless 
protein, but is temperature-dependent in its absence. Twin of eyeless acts also 
in parallel to the eyeless gene and exerts functions that are partially 
redundant with those of Eyeless, while Eyeless is mainly required to prevent 
early cell death and promote eye development in eye-antennal discs.