Development of the vertebrate eye requires a series of steps including 
specification of the anterior neural plate, evagination of the optic vesicles 
from the ventral forebrain, and the cellular differentiation of the lens and 
retina. Homeobox-containing genes, especially the transcription regulator Pax6, 
play a critical role in vertebrate and invertebrate eye formation. Mutations in 
Pax6 function result in eye malformations known as Aniridia in humans and Small 
eye syndrome in mice. The Drosophila homologue of Pax6, eyeless, is also 
necessary for correct invertebrate eye development, and its misexpression leads 
to formation of ectopic eyes in Drosophila. Here we show that a conserved 
vertebrate homeobox gene, Rx, is essential for normal eye development, and that 
its misexpression has profound effects on eye morphology. Xenopus embryos 
injected with synthetic Rx RNA develop ectopic retinal tissue and display 
hyperproliferation in the neuroretina. Mouse embryos carrying a null allele of 
this gene do not form optic cups and so do not develop eyes. The Rx gene family 
plays an important role in the establishment and/or proliferation of retinal 
progenitor cells.