Rx is a homeobox‐containing gene that is critical for vertebrate eye development. Its expression domain delineates a field of cells from which the retina and the ventral hypothalamus develop. The 5′ upstream regulatory sequences of the medaka fish Rx gene are functionally conserved during evolution to a degree that they direct gene expression into the Rx‐expressing field of cells in mice. Using these sequences, we made a Cre line that can be used for inactivation of gene expression in the developing retina. genesis 44:361–363, 2006. Published 2006 Wiley‐Liss, Inc.