The Drosophila compound eye is specified by the simultaneous and interdependent 
activity of transcriptional regulatory genes from four families: PAX6 (eyeless, 
twin of eyeless, eyegone), EYA (eyes absent), SIX (sine oculis, Optix) and DACH 
(dachshund). Mammals have homologues of all these genes, and many of them are 
expressed in the embryonic or adult eye, but the functional relationships 
between them are currently much less clear than in Drosophila. Nevertheless, 
mutations in the mammalian genes highlight their requirement both within and 
outside the eye in embryos and adults, and emphasize that they can be deployed 
in many different contexts.