The key steps in the evolution of full trichromatic color vision in primates include duplication of the ancestral pigment gene to form the L and M pigment gene array on the X chromosome, mutually exclusive expression of the L and M pigment genes in cone photoreceptors, and formation of a retinal mosaic with randomly distributed L and M cones. Previous work using transgenic mice has indicated that a locus control region adjacent to this array of genes plays an important role in their mutually exclusive expression in respective cone cells (Smallwood et al., 2002). However, the mechanism by which this is accomplished is unknown. We searched for a cellular model system to investigate the mechanism of this mutually exclusive expression. We previously showed that the undifferentiated human retinoblastoma cell line WERI expresses L and M cone opsin but not rod opsin genes. We now show that WERI cells express the L and M pigment genes in a mutually exclusive manner, in that either L or M pigment mRNA is expressed in a single cell. Importantly, clonal analysis showed that single WERI cells that express either L or M generate, upon cell division produce, a mixed population of L- or M-expressing cells. These results indicate, first, that cell division resets L or M pigment gene expression, most likely due to disassembly and reassembly of LCR-promoter DNA-protein complexes during cell division. Second, a retinal mosaic with near-random distribution of L and M cones may have been generated automatically after duplication of the ancestral gene to form the L and M pigment genes. Third, determination of L and M cone identity may not require external molecular cues during differentiation, and is consistent with the idea that L and M cones are not intrinsically different.