Deletion of sequences 5' of the human red and green pigment gene array results in blue cone monochromacy, a disorder in which both red and green cone function are absent. To test whether these sequences are required for transcription of the adjacent visual pigment genes in cone photoreceptors, we produced transgenic mice carrying sequences upstream of the red and green pigment genes fused to a beta-galactosidase reporter. The patterns of transgene expression indicate that the human sequences direct expression to both long and short wave-sensitive cones in the mouse retina and that a region between 3.1 kb and 3.7 kb 5' of the red pigment gene transcription initiation site is essential for expression. Sequences within this region are highly conserved among humans, mice, and cattle, even though the latter two species have only a single visual pigment gene at this locus. These experiments suggest a model in which an interaction between the conserved 5' region and either the red or the green pigment gene promoter determines which of the two genes a given cone expresses.