By in situ hybridization, Higashide et al. (1996) demonstrated that the HRG4 gene is expressed specifically in photoreceptors, both rods and cones, in human retina. In rat, the authors observed high levels of RRG4 expression in the outer retina beginning around postnatal day 5, when the photoreceptors begin to differentiate, and expression increased rapidly to reach the adult level by postnatal day 23